Why would a custom order need more capacity? Either a buy a standard model or I buy a custom model. The assembly line is basically the same, just a few differe

Why would a custom order need more capacity? Either a buy a standard model or I buy a custom model. The assembly line is basically the same, just a few different input parts. The assembly line will be rolling out the same number of cars per day. The difference being my custom order got sent to the factory and the next car started on the line was my car.