The fish you see is not the fish you get - a pair of related Orb Vallis and PoE fishing bugs

[LordPreda]

Basically the fish that we see in the lakes of Orb Vallis are most often Magnificent-grade, but after we catch them they're revealed as "Basic" or "Adorned". This is a bit frustrating when you're trying to decide between two fish and one will get away.

Also, on Plains of Eidolon, I have noticed that Large Goopolas will always be labeled as Small in the catch screen. Proof below.

^ this here happens for EVERY large Goopola. I haven't noticed it for other fish but at least the Cuthol seems to be labeled properly.