In 2006, when the pay rates were negotiated, Frontier's fleet was a bunch of (12?) 318s and the rest were 319s. In late 2006 management announced "conditional orders" for 320s, with delivery several years out, as the only growth on the books. 321s weren't even on the RADAR. We were losing money, so future growth on bigger airplanes was difficult to square with the reality on the ground.

Management had a decent argument for paying less for smaller jets if we wanted to have higher pay for larger planes. Planes that may exist in the future - or not. Since larger equipment was pie-in-the-sky at the time, and we'd take a pay cut for flying aircraft that actually existed at the time, all of the 320 variants were included under the same pay scale.

Hindsight being 20:20 we'd have done things differently if we knew where we have ended up, but that's how the situation we have now came to be.