weena makes good points, and you are right, the stamina probably does play a role.
however, with such a small sample size, you don't need any other factors to explain it. estimating your C free throw shooting at 70%, which is probably generous if incorrect, puts the odds at:
(33 choose 17) * (.7^17) * (.3^16) = 0.011684344
or 1.2%, which is quite likely when you are talking about the ft performance of a single player in a domain of something like a hundred thousand players.
edit: i copied the results out of google, that is why there are the useless decimal places. for those who don't know, google has a very useful built in calculator that is poorly documented, but it is quite powerful and convenient.