This is a common problem that results from retroactively changing the Customer tax status to non-taxable sometime after the initial entry, but not also changing the Vehicle tax status and then creating orders with the vehicle(s).
In ordinary circumstances (normal sequence, linear time), customer records are newly created, inheriting the system tax setting. Then the vehicle records are added, inheriting that customer tax setting.
Because the software must support combinations where one vehicle is taxable yet another under the same customer is not (truck with Agricultural plates in some states, etc), the software behavior is different after the initial entry. The Vehicle setting becomes independent of the Customer setting after the fact.
NOTE: It is possible to change the Vehicle tax flag with an Order ALREADY CREATED; it would be necessary to use Order Options – Tax Rates Applied to fix this order that was already started. The software uses whatever setting it encounters at the moment it was created, not what you do after the fact.