i think this comes down to how different people perceive the world in very different ways... ways much more different than we realize.
for example, here Feynman talks about being able to count verbally in his mind while reading, but being unable to speak (since he is using his 'speaking' to count). and his acquaintance is able to count visually in his mind while speaking, but isn't able to read (since he is using his 'eyes' to count).
These sorts of quirks are fascinating; I tried it and I can count while reading, but can't talk and count at the same time. I can listen to music with words while reading, or while writing, but not while doing math or non-trivial programming, which seem to place greater demands on my parsing center.