How come I can dunk indoors but not outdoors?

[Chiddy2theBang]

When I'm in my schools gym I can dunk easily(100 % sure the rim is 10ft), Then I go home and bring out my hoop(10 ft, I personally measured), I can barely dunk. Whats' up with this?
When I'm in school I can do two handed tomahawks, reverses, off the backboards
But at home I can barely pull a simple dunk, infact sometimes I get blocked by the rim..

I know the rim at school is 10ft, I even got the janitor to measure it for me to make sure.Why can I dunk on one but not the other?

 

[MreSonny]

The ground is not level but the gym floor is.