Some summoners have issues as we’ve discussed in this thread (which you’ve also posted in):
Gar’Nok is one of the ones that has issues. As does Summoner and Giant Spider. The summoned’s level SHOULD be based upon the summoner’s Magic stat. It’s just that some cards aren’t working as they should, and are summoning level 15s with weird stat combinations.
I assume you’re Priest’s Magic was boosted? What level does Abhorrath come in if he’s at his normal magic levels? (or is it a kingdom boost that can’t be turned off?)