Every year we have a late-season stretch where teams which are bad and just decide to park and lose a lot of game. This isn't an anomaly, it's called tanking.
Winning percentage + margin of victory + points allowed per game are not good measures of quality individually, but when you look at it together, I think it is a good barometer of a team's direction. Put together that tells me that a team is beating a lot of opponents by decent margins and they are not allowing them to score a ton of points.
However, I agree with you, the comparison probably isn't comprehensive as I think we should have the same comparison for the Warriors against playoff-bound teams. But when you see that the Warriors are 29-9 against teams that are over .500...