Slow hair growth and Male Pattern Baldness

Several years ago I noticed the hair on the top of my head wasn't growing as fast as on the sides. If I go a month or so without a haircut, the sides will be longer than the top even though they started out much shorter. Some of the hairs on top grow normally (maybe 5-10%) but most of them seem to grow very slowly, if at all.

I've searched around and haven't really found much info on this. Is this a normal part of Male Pattern Baldness or could it be something else? Has anyone else experienced this? Thanks for your help