I agree that stats need context and understanding to really make sense of them. However, you don't typically see bench players leading in on-off stats. You can find a list here:
https://www.basketball-reference.com/leagues/NBA_2026_play-by-play.html There are very few players at the top of that list that haven't started the vast majority of games for their teams.
The reason for that is that on-off aren't plus-minus numbers. Backups who do well against benches don't dominate the list, because they're usually not more impactful players than their starting counterpart. Like Kornet is a very talented player who's done a good (though not great) job this year. He's probably a top five backup center in the league. But he gets absolutely rinsed in on-off because his "off" minutes are when Wemby's on the court, and Wemby is second-best player in terms of on-offs in the entire league. in fact, Wemby and Kornet probably hurt each other's performance in that stat. If either had an average counterpart, they'd score much better.
Both Castle and Fox score out better with Harper than they do in general (plus 6.5 per 100 for Steph and plus 1.1 for De'Aaron) or each other (plus 8.7 and plus 3.9). Harper having great numbers with all of the starters (plus 34.8 with Wemby, plus 17.8 with Vassell and plus 14.4 with Champagnie, which are better than their average stats and better than any other pairing them have except with Victor) seems to indicate that Harper has a lot of success against starter-level lineups. Ignoring that the Spurs' staggering of guards means they all get time against starters and bench players, Harper's numbers are more an excuse for Fox and Castle than they are a sign that he's getting inflated stats.
Teams don't normally perform better with their backups in the team than their starters. I know Manu made that seem normal, but it's actually quite rare, even for sixth-man candidates. Monk was negative the two years he was a 6MOY candidate. Naz Reid was negative when he won in 2024 (though positive last year when he placed fifth). As much as I love Bobby Portis, he had negative on/offs during his sixth-man candidacy. What Harper's doing is not normal, and his bench role doesn't explain it.