>>35870687Starmie probably had single digit IVs, and Misty evolved it when it was barely level 5 thinking it's enough to make it stronk. Misty basically fell for the "evolved = better" schtick.
Staryu was Misty's most used Pokemon because she believed it needs to train harder to be as good as Starmie, plus it had stronger IVs (like in the 20s range at least). Therefore, it's not just naturally stronger it's also got more EVs than Starmie as well. That can easily be a good in-game explanation for Starmie jobbing so hard.