I honestly don't know. I don't even really know much of the context of his testimony other than the fact that MTG called him a liar and that violates the House rules on impugning someone's character. Open and shut, regardless of what you think of Mayorkas' job performance or the truthfulness of his testimony.
I suspect that like most high-level officials in charge of running a large, complex bureaucracy like DHS, Mayorkas is probably doing the best he can and realizes that there's going to be a lot of shit his agency could do better. And I suspect that like most people trying to keep their jobs, trying to convince critics that they're up to the task, he's going to put the best spin on things. He will probably accentuate the positives and downplay the negatives. That doesn't make him a liar. That makes him human like the rest of us.