Enemies Who Were Allies Who Sandbagged You Are The Worst

I've been replaying Tales of Vesperia recently, because I realized I missed a lot of side quests and little scenes the first time through. I'm not sure how I was supposed to know to go back to a particular random village after a given event and talk to one particular person there to trigger the sidequest, but that's what the Internet is for. Anyway, things are progressing fairly nicely there, but that's not what I wanted to discuss today.

Replaying the game has reminded me of a certain thing some JRPGs do that I always find really annoying. That is, when you learn a member of your group is actually a traitor, and have to fight them. And it turns out they are vastly more powerful than they showed when they were on your side. Or they get controlled and attack you. Tales of Vesperia uses both, within a relatively short amount of time, in-story. So the character goes from having maybe 2000 hit points when they were on my side, to 50,000. And they're busting out attacks that are far more powerful than anything they unleashed on my behalf.

I know in terms of gameplay, this is because if they were only as powerful as they were when they were on my side, the fight would be over in 30 seconds. Which would be kind of a letdown after the shocking reveal of their betrayal. And it can easily be argued they're ramping up the strength of their attacks because they're either being forced to, or because they're being more reckless with themselves. At the same time though, I think about all the times our group lost fights and had to try again, or nearly lost fights. Or even just times that character was knocked unconscious/killed and had to be revived. Where was this extra power then? They were that committed to maintaining their cover?