The difference is I've never heard anyone say "quarter til ten" before so it sounds very strange. As for it being a regional difference I guess it depends what region of the world you were raised in the south of!
What? That must be a bavaria/switzerland/austria thing. I never heard this. hearing quarter ten (Viertel zehn?) i'd assume 10:15 and most definitly not 09:15 0.o
I'll stay with quarter before ten for (viertel vor zehn) 09:45 and quarter after ten (viertel nach zehn) for 10:15.
Haha I like that take on it. Idk, that's just always how I've always said it. That is, when I don't say 9:45 like a normal human. Northeast US, if that makes a difference