Probably a combination of a number of things. Jersey Royals are meant to be a variety of spud called International Kidney. If they are grown on Jersey they are known as Jersey Royals and traditionally they were one of the earliest new potato crops. Spud harvests head northwards as summer progresses, there used to be a sequence of new potato harvests geared up to get maximum profits for whatever area could produce the earliest crop so, roughly the sequence would go Egyptian, Cypriot , Spanish/Baleric then Jersey and then Cornish.
So, spuds planted nearer the Equator were the first to be harvested and this area of maximum opportunity would move northwards as Spring/ Summer progressed. Unfortunately, this has become a bit of a gold mine so spuds are either given extra water or grown in polytunnels or under fleece to produce earlier and bigger crops. If you over water you get more spuds but with less flavour.... spuds are sold by weight rather than by quality ergo if the growers pump them full of water they gain sales at the expense of taste. I have grown International Kidney potatoes and I found them quite boringly crap, I find Rocket or Foremost a better variety but to be Jersey Royals, they have to be International Kidney and then.... overwatered to make the most money they can.
They don't call you vegster for nothing.
Would certainly explain why the Tesco ones I had earlier this week were, just as per the OP, nothing much different to ordinary new potatoes.