I'm guessing it's because Chet's Offensive Rating is 124 and Wemby's is 104. (League average is ~116.) On offense, Win Shares is based on outperforming 92% of the league average ORtg:
https://www.basketball-reference.com/about/ws.html
Because Wemby is below that threshold, he actually has negative offensive WS this season:
https://www.basketball-reference.com/players/w/wembavi01.html#advanced
One of the longstanding criticisms of WS is that it doesn't account for any tradeoff between Usage and efficiency, so Wemby's 31.8% Usage Rate doesn't really get him anything extra aside from a higher multiplier to his per-possession ORtg gap vs 92% of average (which actually hurts because he is below that threshold). Because of this, high-usage, low efficiency players like Wemby are always undervalued by WS and low-usage, high efficiency ones like Lively are overvalued.
eRAPTOR has a bit of this problem, too, as it views Wemby as a -2.3 on offense (BPM has him as a +1.5). Although with eRAPTOR I think it's actually more inferring that Wemby must not be having all that great of an impact on the offense when the Spurs' ORtg is actually higher when he sits:
https://www.basketball-reference.com/players/w/wembavi01/on-off/2024
eRAPTOR is looking at him and saying "hmm, the league's 27th-best offense actually gets better when this guy sits, and his individual efficiency is way below average... are we SURE he is a good offensive player?"
Is that right? I think something like BPM will end up being predictive of Wemby's future value. But it's always tough to measure the present performance of players who are clearly good enough to get their own shot whenever they want at a young age, but have subpar efficiency and turn the ball over a lot. Are they helping or hurting? Truth is, it doesn't really matter; the 2024 Spurs were never about the present day anyway.
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