If my hearing is above average, why do I have difficulty understanding speech on the...?

The very problem might be your love of reading, writing, and languages -- perhaps as a child you spent more time with book and pencil than you did interacting orally with other children and so never developed the spoken-speech-processing areas of your brain as much as most people do. In any case, you’ve made some excellent speculations; probably you just have a mild neurological deficit in that factor, no doubt more than compensated for by the other interests and talents you mention. i doubt that it’s anything you’ll be able to do much about at age 30, but i suppose you could visit a neurologist. Best might be if you get comfortable with asking people to repeat things, and try to learn not to let it bother you. (Easy to say, hard to do, i know -- but good luck!)