The only thing I can think of is maybe the 8700 has better binning and is able to hit its turbo speed while putting out less heat and so is able to maintain its

The only thing I can think of is maybe the 8700 has better binning and is able to hit its turbo speed while putting out less heat and so is able to maintain its turbo speeds for longer than the 8700k. I think it is more likely to just be testing margin of error