I found the same issue during a triathlon this past weekend. Top + average speed and distance were higher on the Cateye (and slightly higher than the course distance).
If I had to bet, I'd go with the Garmin as far as accuracy. If the Cateye is just slightly off, each mile will add more to the discrepancy. I don't ever recall GPS telling me I'm 1.5 miles from where I really am. (not sure if it's a fair comaprison...but you know what I mean)