It's simple. People never notice those things that hairwearers seem to obsess over.

I work closely with about a dozen people in our office and if one of them had hair that never got longer, I'd never notice that. Why would anyone notice?

If you suddenly went from having hair to having a shaved head overnight, that's the thing people notice. Or vice versa-- very short to much longer, dramatic changes. But worrying about explaining the fact that it doesn't seem to grow? Easy. Your dad/uncle/neighbor is a barber and you're in there a couple times a week for a free trim.

And how to go from thinning hair to a full head? Do it while you're on vacation from the job, school, etc. People don't store that info in their brains for very long. Grow a beard before you add hair, then shave it off when you do. People notice the missing beard and not the additional hair. Tricks like that.