That's interesting. Can anyone verify (or counter-verify) that your actual 'effective level' of fishing has a bearing on what you catch?
At first, I thought that this wouldn't be true. It doesn't seem consistent with the way that Blizzard has done things in the past.
However, I can confirm that the little evidence I have regarding catching the quest items for the fishing daily quest does support this.
Both the Crocolisk quest and the Zangarmarsh shrimp quest completed significantly faster on my characters with a higher fishing skill. Oberweiss, for example, got 2 shrimp in every bloated fish, and every cast was a bloated fish at skill 497 (used the quest reward hat, and too lazy to switch it out, heh). However, Ober usually got ~1.5 shrimp in every bloated fish, and also caught 3 Spotted Feltails during the quest at a skill of 405.