Because ERA is a bad stat over a single season sample. Young pitchers improve year to year but it usually isn't by huge leaps and bounds like people think, the underlying peripherals usually show slow growth the ERA just jumps all over because well it isn't a very useful stat without a large sample. Volquez pitched like a 4 ERA pitcher most of last year, Cueto was over a 4 ERA. Volquez had an xFIP of 4.02 last year and got steadily worse as the season went on, he has an xFIP of 4.80 this year. I don't think many projected Parra to beat his xFIP from last year which was 4.03 so not sure where this big growth for Parra idea comes from. Cueto had an xFIP of 4.62 last year and has a 4.44 so far this year. He has shown some growth but the 2.53 ERA is almost completely a product of his 83.9% strand rate and .231 BABIP against. It just isn't supported by his peripherals at all. Parra had an xFIP of 4.03 last year and has a 4.93 so far this year with that terrible spike in BB/G mostly to blame. The nice thing about Parra is the high GB%. If he can get those walks back down he really does have the perfect skillset, LHP with high GB% and good K/9. He can even live with a BB/G over 3.5 and be somewhat successful but over 5 is going to cause problems.