Is your spark plug socket hard to remove after installing the spark plug? Mine has always been a big effort to pull that socket off the plug.
I was using an old Sears Craftsman 5/16 spark plug socket which is probably 50 years old. It turns out the OD of that socket is 0.888" and it is binding on the aluminum in the spark plug valley.
I now used a 16 mm long socket which has an OD of 0.858" and it just pulls right off with no effort.
Not sure if Craftsman has changed the OD now but I bet this also affected my torque of 15 foot/lbs.