Well, I think that for Dutchies, the discontinuity in Dumoulin's performance is harder to perceive as most Dutch media already "predicted" that Dumoulin would develop into a GC rider.
For instance, in a 2013 (!) article by the Dutch National Broadcasting Foundation (NOS), influential Dutch cycling reporter Mart Smeets already described him as a future Tour de France GC contender. Iwan Spekenbring, manager of Dumoulin's team Argos-Shimano at the time, reacted to that by saying that "he [Dumoulin] has a profile that fits the TdF". Dumoulin's reaction at the time was "I'm not a GC rider for a three week tour. I'm not the greatest climber in the high mountains.". However, he was not fully conclusive, saying that "He would let himself be surprised by him".
Source:
http://nos.nl/artikel/589422-2013-en-de-doorbraak-van-dumoulin.html
To add to that, Brailsford once stated during a press conference in 2014 that he was convinced that Tom Dumoulin was a great talent and that Team Sky had what it would take to develop him into a potential winner of a GT.
Source:
http://www.wielerupdate.nl/wielernieuws/29255/team-sky-gecharmeerd-van-groot-talent-dumoulin/
If you search for terms like "Tom Dumoulin ronderenner" including only articles prior to this Vuelta, then you'll find a lot of Dutch articles predicting a great GC future for Dumoulin, even as early as 2013.
So, for the casual Dutch cycling fan, his performance during this Vuelta does not come as a total surprise, not in the least, as the Dutch media have been speculating about him being a potential GC rider in GTs for years. That there is no actual performance pattern leading up his current Vuelta performance is irrelevant to them, as they
know he had the potential due to all the speculative articles in Dutch media.