New = low APR
Used = higher APR
Last year I went for a 1 yr old (ex demo) van for this reason. Higher Depreciation vs higher APR over 4 years.
Probably works out that way as the manufacturers do deals to secure new vehicle sales.
At one time when the cash back deals were popular, different deals were put together to 'float different people's boats.' Cash back was a manufacturer led additional discount which allowed them to not change their retail pricing structure in 1999 when there was the 'rip off Britain compaign'.
The same cash back deal was disguised in a variety of ways. It might be in the form of interest free credit, cash back, pay no vat, free insurance, free fuel and servicing for a year, or a mixture of some of them.
APR doesn't actually mean anything to you as a buyer BTW. You need to know what the flat rate of interest is that they are charging you. If you are paying 5% flat pa over 3 years you are paying an additional 15% interest on what you borrowed. Easy! If you take that same flat rate over 1 year, 2 years and 3 years and apply the APR formula, you will get a different APR for each period of time.
This means that when you think you are getting a lower APR, you may actually be paying a higher fat rate of interest.