One of the chefs on a TV prog said the other week that they're no longer grown in the traditional way, which apparently involved sea-weed, and that's why they no longer taste as special.
A friend from Jersey told me that to cope with the increased demand for Jersey Royals they are now being grown on land which has not had the same traditional treatment as the 'proper' Jersey Royals. That is why some are good and some not so good, it depends on the 'terroir'.
Early French spuds from Ile de Ré are good and the 'Ratte' variety from Le Touquet (possibly available in UK )are very good too.
I think it was waitrose who said that the jerseys that are grown for them are still grown traditionally on the traditional land...but I can't now find the quote