Grr. Would be nice if BBall-index would, oh, I don't know, actually define "on-ball" and "off-ball" gravity somewhere on their webpage. They define "3PT gravity" but that doesn't have an on/off ball component. Furthermore, they imply that negative numbers, not positive numbers, are good, but give no idea what the values are inferred to mean.
If I have to give a guess based on that, this data means that the Kings ball handlers attract among the highest amount of defensive attention in the league, while their off-ball players attract among the least. This makes some sense - the ball is in Fox's hands as a drive threat or in Domas' hands in the paint quite a bit so on-ball defensive attention is what we would expect, and I guess that would detract from off-ball attention. But I would expect the Warriors to have high off-ball attention (not the lowest in the league), and that interpretation would suggest that the Spurs are among the most closely-guarded teams in the league when you combine on- and off- ball, but that the Sixers are the least-guarded team in the league. That doesn't seem to track.
So...I'm not sure I can really make heads or tails of this, but hey, the Kings are up in the northwest corner!