My neighbor is not schizo after all

Subtitled: I am not very observant

So I have been vaguely mystified by our upstairs neighbor, a single guy who is probably in his 30's.  Some days I see him and he's saying, "Hey, how are you?  Have a nice day."  And other days he will barely grunt if you greet him.  But whatever.  I figure maybe some days he's well rested and other days he's tired.  Today I was getting out of my car when he was getting into his car in the adjacent parking spot.  And it's not "he".  It's "they".  So I actually have two upstairs neighbors.  They are clearly not twins, but they may well be brothers.  Same basic height and build and look.  So we have friendly neighbor and grumpy neighbor.  Confusion abated.  It took me how many months of living here to figure this out???