One reason highly intelligent and resourceful people with disabilities sometimes go undiagnosed until adulthood is that disabilities can be a matter of degree rather than kind.
For example, most people have an easier time following conversations at home without background noise than at a crowded bar with music blaring in the background. People with auditory processing difficulties differ in how much this situation affects them. The average person can understand and participate in conversations at a party (although granted, these tend not to be highly complicated). A person with auditory processing difficulties can stand equally close and pay rapt attention and may still not be able to understand such a conversation well enough to participate.
Similarly, most people find phone interviews for jobs more difficult than in-person ones. In phone interviews, one loses all visual cues to the other person's reactions--facial expression, gestures, small involuntary movements, and so on. Less obviously, one also loses the power to influence (or be influenced by) others through visual cues; other people can't feel the quality of your eye contact or synchronize unconsciously with the rhythms of your movements. In other words, some factors that make a person charismatic in person may not work on the phone. Many people probably know all this on some level, and feel nervous when they have to interview by phone. People with auditory processing disorders have even more difficulty because they have more difficulty coping with the slight distortions in sound quality and pitch imposed by the phone; they can't even rely on auditory cues to make up for the lack of visual ones. People with social or communication disabilities have even more difficulty than the average person too. The number of social cues they can perceive or interpret even in person is smaller than average; depriving them of even more cues by putting them on the phone may blind them still further.*
The basic principle here is the signal to noise ratio discussed in an earlier post. Everyone needs a certain amount of meaningful signal from the environment and a low level of interfering "noise" in order to understand the world. That "noise" can come from the external environment (e.g., background noise from TV or dishwasher; distracting glare on the windshield) or it can come from one's own brain (variability in neuron firing rates and the like). People with disabilities have a higher level of "noise" than the average person**, so they may need more signal to understand what's going on. They may not have quite enough "signal" in normal situations, but the difference may be small enough for them to compensate. But when "noise" levels are high enough to give neurotypicals difficulty, people with disabilities will have much more difficulty functioning.
If you look at disabilities as a "signal to noise" issue, it becomes a lot more obvious why an autistic person would have difficulty understanding subtler social signals, but can comprehend more obvious and directly presented ones. Or why they might have particular difficulty understanding lying or trying to cover up one's emotions--people with something to hide produce interfering signals or "noise" to mask their underlying emotion, and autistics may not be able to cut through the "noise" to see the subtle signs.
Thinking about signal and noise also makes sense of the variability in functioning many disabled people experience. Autistic people seem to have particularly extreme fluctuations. For example, on one day some may be able to speak just fine, on another day they might lose the ability to speak entirely. (Not everyone on the spectrum has such extreme fluctuations; those who would be considered "lower-functioning" may experience more extreme loss of skills, particularly under stress). Recall that tasks that seem incredibly basic, including speaking, require a person to have many other sub-skills and information about the world. In some situations, a disabled person may have sufficient signal coming from the environment and low enough noise from the environment and their own brains that they can exhibit a skill. Stress may raise the internal "noise" and environmental distractions the external noise so much that they can no longer perform one or more essential sub-skills. I'm not sure what sorts of functions are disrupted when a person loses the ability to speak. This would be a great area of research--hopefully it exists right now and I just don't know about it yet.
So, if you're neurotypical and trying to understand why a disabled person in your life just doesn't "get it," imagine your own confusion and frustration in the loudest, busiest environment you've ever experienced, then multiply it several times. You know this experience, too--disabled people just deal with it a lot more often.
*Not everyone on the autism spectrum responds in the same way to the phone. People who find talking to others in person visually overstimulating and are exhausted by demands to look others in the eye and appear "normal" may find the loss of these demands refreshing enough to make up for the lack of information imposed by the phone.
*Some research suggests a higher level of "brain noise" and other research indirectly suggests it by indicating that an individual person's variability in responses is much higher if they have disabilities, particularly ADHD. More obviously, they may be more sensitive to, and aware of, environmental "noise" than the average person, and if they have attentional difficulties, they may be less able to filter it out.