Theirs this person who lives on the 20th floor of a high rise building,
But for some odd reason when it's sunny out the guy goes to the 10th floor
And walks up the rest of the way to the remaining ten floors,
Odd thing is that, when it's raining he goes directly up to the 20th floor, that's where he lives... why is that ?

Because he's a child or a midget who can't reach the 20th button on the lift. If it's raining, and he has an umbrella, he can use that to push it.