Incorrect. The GPS is very accurate, particularly over some distance. (i.e. immediate speed reading can fluctuate with reception, but over a number of miles its very accurate.)

The Cateye is only as accurate, as the programmed wheel circumphrence. If you really want to dial that in, you need to do a rollout, with the bike loaded with your weight, and the tires at the same pressure you are riding.

Absent such a rollout, just using the numbers off a chart will not be nearly as accurate as GPS.