It's just a cultural thing. When two communities/cities/states/countries are separated from each other in some way (obviously, the greater the separation, the bigger the difference) they develop differently.
A funny example (with serious consequences if you misuse the words!)
fag mainly in BrE
fag mainly in AmE
=homosexual <OFFENSIVE, INFORMAL>
fag mainly inAusE
=in school, a junior who performs certain duties for a senior
Gotta be careful when using words!
- For Teachers