I am no expert but I think when hair is thinning, hair follicles cycle through short growth cycles, thus hair on male pattern baldness affected areas appears to be thinner and shorter as they grow and shed at a faster rate than normal hair does and eventually die..
I buzzed my head about 6 months ago, and I can confirm that MBP affected areas grow slower than the non affected areas. This is a personal experience, but I don't think it would differ from person to person.