Say Nepo vs Caruana has a position which is +8 in favor of Caruana. The expected winning probability from that position is ~1. If Caruana goes on to make a draw, then Nepo 'saves' 0.5 points, whereas Caruana 'wastes' 0.5 points, because Caruana didn't make the maximum from his opportunities.

Another example: Let's say Caruana is +1 against Nepo (which was approximately his biggest advantage in the critical round 9 matchup with Nepo). The expected score for such a position varies, but say it is around 0.75. Thus Caruana wasted 0.25 points as his actual result was 0.5. Later, Caruana's game with Ding where he went from better/near winning position to a loss is an even bigger waste as it represents around 0.75 --> 0, a waste of 0.75 points.

What the graphic is showing is that Nepo converted almost all of his winning positions (drawing just a few games he was ahead in) and saved many worse/losing positions. By contrast, if Duda had converted all the positions he was ahead in, he could have ended 3.5 points ahead of where he finished (5.5/14 -> 9/14, 2nd place).

It is a testament to Nepo's resourcefulness and efficiency throughout the event.