Robertson has been consistently quite good for years. He did alright in a relegated Hull side and then was their best player arguably when they got promoted. He scored against England all the way back in 2014.
It's not like Flanagan, where a nobody had one really good season out of nowhere. Robertson has clearly been a quality player for years, but has just went from playing with Hull, to playing under a world class manager with a world class side and logically went up in his own level.