You're correct -- this isn't normally how credit computations work. the credit amount is tentative because the manufacturer or domestic distributor of the vehicle will tell you the amount of the credit allowable, based on your specific make, model, and model year. This isn't a typical procedure for other credits.
According to Line 9 in the Specific Instructions for Schedule A (Form 8936) in the Instructions for form 8936:
Tentative credit amounts for new clean vehicles are provided to the purchaser by the seller at the time the vehicle is sold, and later forwarded to the IRS. Generally, this amount will be the maximum credit amount listed in the seller’s report for the vehicle.
Since the seller is using the maximum allowable amount, it's possible that the actual amount could be lower. However, you should use this number when filing Form 8936 to claim the credit. If there should be a change, you'll presumably be notified, although no guidelines appear to have been established.
