Why are row numbers 10 and 11 skipped on Frontier's A-319?
Airline row numbering has more to do with fleet consistency than sequential numbering in one airplane type.

Head over to SeatGuru and look up Frontier. They fly 3 types of mid-size narrowbody - A319, A320, Embraer E190 - and not surprisingly row 12 is an exit row in each one. The E190 and A319 don't have row 11, only the A319 is missing row 10. 

It's the same reason 2+2 seating skips seats B and E ( AC DF ), where 3+3 seating goes all the way across ( ABC DEF ). Widebody aircraft have a different sequence because it doesn't need to match - the same cabin crew will rarely work both major classes and they don't tend to run regional airlines on widebody (long-haul) routes.

Look at Air Canada's fleet - row 18 is the beginning of economy and an exit row in almost the entire widebody fleet (the exception is probably historical).