No, your stats in arena are the basic level 100 ones the ninja owns + initiative. The 10% thing was there for a very short time after 2.0 introduction and was removed after 1-2 months due to a ton of complaints.

In arena the combo rating of a ninja is 0, so just have the basic chance to start a combo (that's 30% i think), unless we talk about special standard attacks.

The water whip of azure fang clone is one of these special cases since have an enhanced basic chance to land and start a combo.

I've ran mei, mabui, tenten gnw, azure fang team for 6 months in arena and i can tell you you were just unlucky (and that the guy you met used a subpar set since azure fang necessarily have to run healing jutsu as standard in that setup, since a reflected acupuncture without a 2nd healing standard is very likely sure defeat). Personally speaking i think quite the opposite about mei and by experience i'm almost induced to think her chance to apply acupuncture and start a combo is, if possible, lower than the basic chance, in the sense that is artificially lowered to 15-20% to reduce the otherwise op set she owns by herself.

Every person that experienced this team in arena can tell you that took for almost granted after tenten moved and didn't start the combo that mei wouldn't start it and thanked god when this happened while instead cursed him every time the clone didn't start it in the same case :-).

About azure fang 3rd standard i can assure you there's no enhanced chance, otherwise beating some ninja exams wouldn't have required me 100+ tries when i was low level and bp :D.