That ETRTO/ISO chart has always bugged me a little. It's one thing to debate whether 2.2x or 2.4x the inner rim width is too wide for a tire, but they are inconsistent at applying any formula I can think of. If 57mm is okay on a 25mm rim, and 50mm on a 21mm rim, why not a 54mm tire on a 23mm rim? If they approved an 18mm tire on a 13mm rim, why not a 32mm tire on a 21mm rim?

We understand that these are guidelines and that we can go outside of them if we're aware of the side-effects, but guidelines themselves ought to be internally consistent!