My hypothesis is that the middle digit is the fleet designation. This isn't unlike when it comes to destroyers, that "16" and "17" denotes South Seas Fleet, like 167, 168, 161, "13" and "15" denotes East China Sea Fleet like "131" and "153", and "11" and "12" is North Seas fleet, like "116", "117" and "121".
So its possible that "12" starts the carrier line for the South China Seas, and "16" starts the carrier line for the North Seas.
Think of it this way, if 002 becomes "12" in South China Seas appointment, 003 becomes "17" if she is commissioned in the North Sea Fleet, and "13" if she is commissioned in the South China Sea. I don't think there is any room for a carrier in the East China Sea fleet due to the lack of port facilities, so poor fleet, they have been a bit neglected lately.
My new hypothesis for the carriers is four for South China Seas, and four for the North Sea. 12, 13, 14, 15 is for the South China Seas; 16, 17, 18, and 19 is for the North Sea Fleet protecting the Capital region.