Part of that was due to Girardi playing sheltered minutes in Tampa. Nemeth was already sheltered in Colorado and their fans still hated his guts. So I don’t know.
Take one player who seems to "play a steady game, doesn't make mistakes" but stats over a long period of time show that when he's on the ice his team gives up more goals than expected, consistently and all the time no matter his usage. The easy explanation is that while he doesn't make obvious mistakes, his ability to actually defend and to transition the puck is very lacking and he is actually putting his team at a disadvantage because he has a negative impact on goals against.
Another player who "makes mistakes sometimes" but who consistently shows to be a positive for his team when on the ice defensively (fewer expected goals against), and the easy explanation is that he is good at defending and transitioning the puck but occasionally makes noticeable and obvious mistakes that leads to chances or goals against. However, over a long observation time, those occasional mistakes result in fewer goals against than the first players consistent but poor defensive game.
Basically, a player like Nemeth probably makes mistakes sometimes that look bad, but his overall impact is good because the mistakes don't happen all the time and he otherwise is quite good at defending as born out by years of stats that show a positive effect on goals against.
The players who don't make mistakes and also consistently grade out well defensively are players who are generally top four guys (or undervalued) not bottom pair defensive guys
Nemeth will make mistakes, but his overall impact defensively will be a plus compared to a player who looks like they don't make mistakes but are actually bad at defending and give up more goals when they're on the ice