If you read the descriptions for the different scores, most TMs would fall into E or IE. U is you don't do much, if anything, well and you're probably being performanced out. IE is you do your job most of the time, but there's still some stuff you don't do well. E is you do your job and do some extra stuff without being asked every now and then. Ex is you do your job very well, many times you go out of your way to help out in other areas making significant impacts in the store. and you don't need much follow up. Basically, you know what's important, get your job done, and make your TLs and ETLs workload lighter. O is superior results are delivered and sustained over a long period of times. Results and the methods used to achieve them go far above and beyond the call of duty.
Everything but O is kind of the main gist of the blurb describing the grade. O is pretty close to what the blurb actually says. With the way the scores are worded, O should be seen very rarely, EX is really the best anyone can hope for, and most TMs are going to get E followed by IE.