Little thing called Law of Averages. He is playing really well because he is being really efficient, but there is no chance Terry continues shooting 50% from the field and 54% from 3.
Jason Terry wasn't a bottom of the league player with us, despite what a lot of people will tell you. He was a 10ppg, 2.5apg guy with 43% shooting and 37% from 3 (which is right with his career efficiency).
Just so happened that the Celtics needed a little more out of those minutes, and also, for some reason fans thought he would be the new Ray Allen. Ray spoiled us way more than anybody will admit. Regardless, Terry's shooting is the stuff of legends. He somehow has shot in between .362 and .379 from 3 in each of the past 7 consecutive seasons. I dare you to find another shooter with that little variation.
Chances are that's the ballpark he'll land in by the end of the year. And if not, all that means is he finally re-joined the ranks of every other pure shooter who have up and down shooting percentages.