Not a surprise at all - as an Australian I have seen the level of competition here.
There is a serious lack of size, athleticism and talent here compared to in the US. Ifyou imagine a lineup of David Lee / Dajuan Blair / Keith Bogans / Jason Terry / Jameer Nelson then that would look a lot like the typical NBL player at each position in terms of size and athleticism.
Needless to say, by NBA standards all of those guys would generally be considered below average in terms of size and athleticism at this stage in their careers, so if you aren't familiar with the NBL that should give you some idea of how much advantage US college and pro players have over guys in the NBL over here.
NBA scrubs who can't get on a team tend to absolute dominate the NBL. For example, Josh Childress was the best player in the NBL last season, ranking first in scoring, 1st in rebounds and 5th in assists and generally dominating the league. This is the same Josh Childress who would (at this point in his career) really struggle to find a spot on any NBA roster.
Just to put it into perspective, in the NBL in 2014/15:
* Only 4 players averaged 18 PPG or higher
* Only 1 player averaged over 8 RPG
* Only 1 player averaged over 5 APG
Even mediocre college players (guys who aren't good enough to get drafted) who come over here tend to immediately become a top 2 player on whatever team they play on.
Thornton was ranked 17th in the NCAA in scoring (20 PPG) which ranked him above some very highly rated scorers including RJ Hunter, D'Angelo Russell, Frank Kaminsky, He also shot nice percentages for a PG, shooting 45/40/83. The guy can score.
He's also got decent size for a PG, even by NBA standards. He's listed at listed at 6'4" @ NCAA, 6'3" / 190 lbs @ CBS sports, and 6'3" / 177 lbs @ draft express. He also has a 6'8" wingspan, which is only 1" shorter than Rajon Rondo and is relative to his height.
On top of all that, his athletic measurements rank him one of the best (if not the best) pure athletic in this year's draft. His max vertical of 43" is insane, and his sprint & lane agility scores were off the charts.
I know it's not often that late 2nd round picks end up overachieving and becoming good NBA players, but if there is any late 2nd rounder from this draft that has the potential to do that I think Thornton is right up there.
Having a guy with his combination of size, athleticism and scoring ability here in Australia - I'd say he has a very legit shot at instantly become the NBL leading scorer and a top 3 - 5 player in the NBL.