I'm not certain the dealer can artificially massage the residual figure, but by changing the annual mileage you can do that yourself.
For example if you have gone for 10k miles p.a. you could reduce it to 8k miles which puts the residual up. You need to make sure this is what you want to do though, as come hand back time, if it has got more miles on than the agreement states, you end up paying a pretty hefty mileage 'fine'. The opposite is also true, if you want to make sure there is lots of positive equity in the car at the end (to trade in as a deposit) you can put the mileage up, which reduces the residual value.