1. Victoria Justice appeared as different characters on Zoey 101, iCarly, and, of course, Victorious. This would lead us to believe that each show took place in its own universe.
2. BUT she played Tori Vega in the "iParty with Victorious" crossover episode of iCarly, which means that those two shows do exist in the same universe.
3. On top of that, Miranda Cosgrove and Jerry Trainor had big roles in Drake and Josh and iCarly, which means those two shows couldn't be in the same universe either. But Drake and Josh was a TV show in the Victorious universe, which we know because the characters talk about it in various episodes.
What all of this means is that there was a universe in which Carly Shay (Miranda Cosgrove) would watch Drake and Josh and see someone who looked exactly like her playing Megan Parker.