I used to think this too. What i then noticed is that when you move the general into the province that you are attacking the number of command stars he has are updated. So if he gets +1 for attacking when outnumbered and he is the only unit in the province being attacked then he gets a bonus command star. The problem arrises when you star piling on more units as people tend not to check back when they have already done something. You may find that this general starts with the 6 or 8 stars that the virtue gives him but as more troops are added he loses one. Bummer I know.
I noticed this because I had a general with specialist defender and just sitting still he had 9 stars. But when I attacked with him, he only had 6. I was a bit shocked and sat down to work it out. It happens more often that you think.