3 of 10 is 30%, not 33%. Or at least it was when I studied arithmetic.
LOL, I think you might have it mixed up...3 out of 9 is 30%, but 3 out of 10 is 33.3333333...%
Go back for more arithmetic.
I, too, was wondering about that. Maybe the three out of ten is for one year, but the 33% is based on multiple years? Of course, if that was the case, I would have thought the author would have noted it.
On the other hand, maybe the simplest explanation is the true one - the author of the article is just bad at math.