I have made quite a few DVD-A discs for play in the car with no problem. I just purchased a commercial DVD-A just to see if there are any differences. When I put the commercial disc in, I have to turn the volume up a lot higher than on the DVD's I make. Is this the norm?

 

It could just be that disk. Typically disks are normalized in some way so that the highest peak comes close, but not over, the highest level the disk can capture. Typically, but not always. Much depends on how much range the producer wants there to be between various songs or sections.