I don't often see C as the average though so that part doesn't bother me. however, the '+' designations are probably valid. as the list mentions at the top, SS was changed to S+ to emphasize less of a gap with S. so you can put the same reasoning for A+ and B+. normally, if you saw Unit A was A tier and Unit B was B tier, it'd be really easy to claim Unit is is superior to B and/or can be used in like 85%+ of situations. however, B vs B+... shrinks to either metagame considerations (more so at least), or something like 60%+ of situations instead. and people wouldn't immediately say B+ outright outclasses B, which is probably the intention