A few things:
There are/were two ways to rate octane-know as Research and Method.
One way, the octane way mathematically calculated. The other way, a motor with a variable compression ratio was used to test a fuel. Basically the motor would run at higher and higher compression until it detonated, and given a rating that way.
If you look at a '70's VW Beetle, which had about 7-1 compression ratio, the sticker under the filler cap said "Use Regular fuel, 91 Octane". Which to Americans now, is contradictory.
What happened is that decades ago, the USA decided to average the preferred US and Euro rating systems. When the new system came out, the pumps would say 'R+M/2', meaning an average of the two.
So, yes, 91 in current US fuel is closer to high test was in the old days.
Ruttly-in response to your comment about people mixing fuels of two octanes to get a higher one, there was a reason.
When the US gradually switched from Leaded fuel (Tetraethyl Lead) to Unleaded, there was a time when stations offered Unleaded, Low Lead, and Leaded.
Only a little bit of lead is needed to boost octane.
So, the gag was to use 3/4 of a tank of Unleaded high test, and 1/4 tank Leaded regular. The lead in the leaded brought the octane of the octane of the unleaded stuff even higher. So, the mix had a higher octane than either of the bases.
I remember doing that for a while
No leaded gas at the pump, except in some Marine or Aviation places, so this is no longer done.