Given that there used to be plenty of room to overclock the cores while still keeping them stable, I think it was more "performance on the table".

You could also get the idea that vendors sometimes make strange decisions which increase neither performance nor reliability.

For example, various brands of motherboards are / were known to basically blow up AMD CPUs when using AMP/XMP, with the root cause being that they jacked an uncore rail way up. Many people claimed they did this to improve stability, but overclockers now that that rail has a sweet spot for stability and they went way beyond it (so much so that the actual silicon failed and burned a hole in itself with some low-ish probability).