Without sweeing the other car and its condition I can't comment, but I bet it wasn't as well spec'd and not in as good condition.
Also buying at auction is always cheaper than private as you buy it without driving, speaking to the owner and no warranty. That is like compariing trade in value's against private sale values. You can't do it.
I mustn't be the only one then as when you have a look at carsales mine is priced well within the market.