According to then Sound Committee Chairman Howard Crispin, in AMA pub SOUND AND MODEL AERONAUTICS A handbook for model clubs, 1991, "Some time ago the AMA Executive Council voted to approve a standard of 90 dBA at nine feet as a recommended figure for clubs to use for operations." It was adapted from a figure used widely in Europe (82 dBA at seven meters) with a change to measurement distance and units, and is essentially the same when the difference in spreading loss due to distance is accounted for.

You aren't losing your mind, but the keyword is "recommended." It may be a reasonable ballpark figure for many clubs sites, but not a panacea. Two most significant shortcomings are 1) not all flying sites are equidistant from neighbors that are potentially impacted by the noise, and 2) it lacks a nexus to cognizant authority. Local (county, city) political subdivisions are the controlling authorities that establish and enforce noise abatement standards, not AMA.