Difference is about 1 question or even 0 questions(depending on how difficult the test was and how everyone else who had the same test scored). From 22-23 its about two questions. Im 2 points from a 23 AA, if i got 1 more bio or orgo or gen chem question right I would have a 23.
In terms of impressive, I'd say a 25-26 AA would be similar. Its hard to say for the lower numbers. I would think a 28 or a 27 are both impressive beyond imagination. Once you get high enough its like comparing a 96 to a 97, one definitely looks better but inherently its the same thing. But 24 would be slightly better than a 23. The falloff is bigger from 22 and down imo.
My biggest gripe is that a 20.6 and a 21.4 both result in 21s despite an entire .8 point gap. Really should include the decimals imo.