I don't think it matters whether a gym has a mobility score higher than the USAG mobility score, or it doesn't. What matters is that the parents and gymnasts understand a gym's philosophy about how it makes mobility determinations as well as other choices and (most importantly) that the parents & gymnast agree with the philosophy of the gym they attend.
I agree with
@CLgym that a big score in Level 3 indicates the gymnast is good at Level 3 (and possibly little else to be known via the score), but at the same time I do think scores and mobility can give valuable information about whether or not a particular gym is (or is not) a good match for a particular family.
Personally, I wouldn't want my child to attend a gym where the compulsory scores on team were very high and the optional scores for team decrease dramatically (or even just steadily as gymnasts move up toward Level 10). That would indicate a gym whose priorities don't match those of my child or our family. But that same gym might be a perfect fit for another family, depending on the reason for the score decline (don't have a big optional team, gym doesn't go higher than Level 7/8 usually, etc, etc). If your child probably won't move past compulsories, attending a gym with a super high scoring compulsory team might be exactly what a family/gymnast wants.