The late Jim Hall of the Sierra Bullets Ballistic Test Laboratory said all ammunition tested at the Sierra test lab was all full length resized.

Jim Hall had a humorous saying about full length resizing.........

"The cartridge should fit the chamber like a rat turd in a violin case"

Bottom line, if you ever saw the case thickness variations that can cause a case to warp and the body of the case become egg shaped. You would understand why giving the case a little "wiggle room" to allow the bullet to be self aligning with the bore is a good idea.