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.
It’s not the math that matters, the matter is what’s the matter with Islam.
Instead of one-third being 33.33333333...% (with an endless series of 3's) it should be exactly 33%. And pi should be exactly 3.