It's a precision illusion. Obviously there's no actual a 100 (or, technically, 101) levels of lovemaking skill that the rater is able to recognize and meaningfu

It's a precision illusion. Obviously there's no actual a 100 (or, technically, 101) levels of lovemaking skill that the rater is able to recognize and meaningfully distinguish. However, having a 6.5 seems like it was well considered and thought of in detail while just 6 looks like sloppy "barely better than absolutely mediocre". So, the person using 6.5 is pretending (unwarrantedly) to be very knowledgeable and precise in these evaluations. This is, btw, a very realistic part - a lot of people do this in real-life contexts, much different from rating girlfriends.