Thats a big difference for the last core and suggests:
The base of the cooler may not be flat
The base may have some material on it
Or the grease is not evenly spread on the base
Or the core is not seated flat in the socket
Or the core is not flat itself
Or the core has a manufacturing defect
It is unlikely to be one of the last two ..
Shutdown, remove the HSF, clean the base thoroughly with a brillo pad and check there are not pits on it ... you could give it a very light sand with a piece of very fine sandpaper. Make sure you then clean away any metal filings carefully.
Clean the core carefully with a couple of cotton buds and some metho / alcohol or if it is the white silicon grease it will come off with no solvent.
Get some arctic silver AS5 or similar quality cpu grease and reapply to the base of the HSF.
Take out the cpu and then reseat it in the socket carefully.
Replace the HSF on top and check the retention mechanism locks down firnly - I normally give it a wiggle after ... others say not to.
Check everything is fine then fire it up and goto the bios and let it settle for a while and note the temps.
Hope this helps.