This is basically my guess. In the event of no mitigations you have to pessimistically flush TLB. In the case of mitigations the flush only has to happen in specific instances, which PCID significantly helps with but requires cooperation from both hardware and software.

So maybe even something like "if PCID is not set, flush the cache" vs "if PCID changes, flush the cache".

I think that would certainly account for this, and they could have improved the performance with PCID by increasing the PCID cache size.