its all to do with the US habit of knocking off the “2” so 29.91 becomes 991 on the RT.... There was an Airprox a while ago in the Shetland Isles between a US crew in a Kimg Air and an ATR. That’s what caused it all, I’m told. The AAIB report refers.
Having said that, I went to SNN recently and the atco asked us what units we were using. Mb obv....! Well, we were tempted to say it for a bit of sport but remained professional with HPa.