Yes, I do think that there is such a thing as a savant ability to take standardized tests.
I'm living proof.
I was what could (only very charitably) be termed an "indifferent" student throughout high school-- and my GPA showed it.  Oh, sure-- I am 'gifted.'  I'm HG, in fact.  But I had a 2.4 GPA.  (Er-- or thereabouts.  I'm not sure, but I definitely had a few grades LOWER than "C" on that transcript.)
But still-- every standardized exam I've ever taken I've hit a home run with.   The same way that you've reported-- the kind of "WOW!!" performance that seems completely incongruous with the rest of the person everyone knows.  National Merit Semi-finalist (easily making the cut), etc.  
My DD is a natural test-taker, too.  She just flies on the adrenaline, I think.  I don't know enough about her internal landscape to tell you what is going on in her head.  But I can tell you that I'm someone who enters a flow state during standardized exams.  I'm just seriously in the zone-- it's showtime, I guess.  Like I'm working the NYT crossword puzzle, or the world's biggest Sudoku or something.  It just FEELS like 
play while I'm doing it.  It's a rush.  I'd take the GRE every day just for fun, basically.  I liked it that much.  
I'm not 
really "that smart" if you were to meet me and talk to me, though.  I just happen to enjoy taking standardized exams, and-- well, whatever a HG person tends to pursue that way, they tend to be pretty extraordinarily good at, too, YK?
Congrats to her on the excellent score, by the way!  
