I wonder in the case of Welsh schools year’s ago (and I’ve heard the same thing) if it was a case of different vocabulary. If I landed in the village I grew up in, spouting on about " a deficiency in part of the internal combustion system" - when the normal lingo might be “the engines knackered”, then I would be regarded as posh.
I wonder if it was more a case of being a bit too good and not having a more basic register to go with it.