Why C4 as opposed to A2?

Although I totally understand why a Class A FM station would want to be able to apply for more power, but why is the FCC stuck on using the "C" designation? C, C0, C1, C2, C3....and now C4? Why not just give the "A" another class? Like A2? Wouldn't that make more sense since C4 would be 12kw at 100 meters.

Here's a list of all the FM classes of stations:
Class A 6.0 kW / 100 meters 60 dBu 1.0 mV/m 28.3 km 16.2 km
Class B1 25.0 kW / 100 meters 57 dBu 0.71 mV/m 44.7 km 23.2 km
Class B 50.0 kW / 150 meters 54 dBu 0.50 mV/m 65.1 km 32.6 km
Class C3 25.0 kW / 100 meters 60 dBu 1.0 mV/m 39.1 km 23.2 km
Class C2 50.0 kW / 150 meters 60 dBu 1.0 mV/m 52.2 km 32.6 km
Class C1 100.0 kW / 299 meters 60 dBu 1.0 mV/m 72.3 km 50.0 km
Class C0 100.0 kW / 450 meters 60 dBu 1.0 mV/m 83.4 km 59.0 km
Class C 100.0 kW / 600 meters 60 dBu 1.0 mV/m 91.8 km 67.7 km

And what's the difference between a C3 and B1?

The number after the class letter designates a reduction in power/height from the base letter. So an "A1" or "A2" would be smaller than a 6kW A. (Canada actually does use an A1 designation.)

I believe that B1 and C3 stations are essentially the same, up to 25 kW at 100 M HAAT, except that the F(50,50) 57 dBu contour is supposed to be "interference free" and is protected as such for B1 stations, while the 60 dBu contour is respected for C3 facilities.

The C4 (up to 12 kW at 100 M HAAT) classification would be protected to the 60 dBu contour and limited to Zone II. I suppose an equivalent B2 class could also be introduced some day, but the interference free / protected contour would have to be determined as well, perhaps at the 60 dBu contour, the 57 dBu contour, or something in between for Zones I/I-A.

--- Casual Observer