Most leases charge interest on the whole value of the vehicle and you pay principal on the value - residual.
Figuring out if the residual is good or bad is tricky but 50% is average on most leases. That is why leases are lower on a monthly bases as you are effectively in a 72 month payment plan but you give up the vehicle. Better off buying at 60 months IMHO or go 72 if you have to but I try to stick to 48 for the best rates.
Ask them about a 72 month lease with a 1 Dollar buy out