It is established in the EU that the X-wings were cutting edge, while the Y-wings were older ships. Though I think it would make more sense if they were both older models, it doesn't make much difference either way. The point is made that there are probably more ships than pilots.

In the radio drama, Luke does a bang up job in the flight simulator and Red Leader recognizes this. It's not the best explanation, but it's probably the best retcon that can be made here. In fact the scene where Red Leader questions Luke's ability to fly the X-wing makes this explanation rather awkward, so let's forget about the Special Editions for right now.