There are not many people buying new cars so used should be stronger, and for the most part this is reflected.
However the way people are valuing used vehicles is changing, the old 'Book' is being replaced by 'Live' systems.
For example, what happens to 4x4 values if it snows?! They shoot up.
But if you were using the old 'Book' the value wouldn't change, hence why traders, dealers etc are making the change.
I believe this is why you will see used car market prices change more regularly, not sure if its a good thing or not though!