It appears a blunder is not necissarily a bad move, but a move that signifigantly drops the ratio

In this game, my opponent blundered his queen. It happens. However, on the last move, before resigning, he also blundered his knight. Curiously, the game did not consider this a blunder as after losing his queen, his chances were very low already. So, in this case, the difference between his position and his position after dooming his knight were so close, that despite it being a bad move, the engine did not bother to mark it as one.

This then gives some insight, an inaccuracy, mistake, or blunder are drops of the ratio by some significant mutually exclusive margin- a low drop, medium drop, or large drop respectively.

The feature is explained here: lichess.org/blog/WFvLpiQAACMA8e9D/learn-from-your-mistakes

But it appears you figured it out very well without reading.

Well, presumably a blunder is still necessarily a bad move...

In the reverse case, in this game taking my opponents knight with my knight won material without giving my opponent sufficient compensation, however it was still marked as a blunder by the analysis as my advantage before taking my opponents knight with my knight was even greater. So in this case the difference between my position before playing knight takes knight and after was so great that even though it was a good move, it was still marked as a blunder.

@acgusta2 said in #4:
> In the reverse case, in this game taking my opponents knight with my knight won material without giving my opponent sufficient compensation, however it was still marked as a blunder by the analysis as my advantage before taking my opponents knight with my knight was even greater. So in this case the difference between my position before playing knight takes knight and after was so great that even though it was a good move, it was still marked as a blunder.

But it wasn't a good move. Your opponent just missed the tactic and didn't punish it. Follow the engine line after Nxd4:

8.Bxf7+ Kxf7 9.Qh5+ g6 10.Qxc5 Nc6

You're up 2 points of material and still better than your opponent, but your king is super exposed and the white queen is a monster. This is clearly much worse for you than if you had captured with the bishop.