Scratching my head trying to figure out the logic of what constitutes F vs YF.

I'm currently driving a bare-bones 2016 Hyundai Sonata that's tagged YF. Last week returned a 2017 Kia Optima tagged F. Optima was slightly better equipped. So a Sonata is technically an upgrade over an Optima?

I typically rent CCARs, which seem a little more logical as the "Y" indicates a higher trim level. Base model Mazda 3 or Sentra tagged as C, whereas a Chevy Cruze LTZ (old style) or fully loaded Corolla Sport is tagged YC.

Any ideas?