The BB's have said that they rebalanced the ratings. In the past, there were many more 5s and 4s, but they said that there are now about 20% of each. So perhaps your player would have been a 4-star in the previous drafts.
And box scores value offense over defense, since steals and blocks are the only defensive statistical categories, while a good defender may cause his man to go 0-10 or better yet 0-3 since his teammates can not even see him because of the close guarding.
Or maybe he just had a good day
.
The box scores seem to be more representative this season. Last season, I think I had a few where the player had less than 10 minutes. This season they are at least over 20 minutes.