Were you under the impression that your truck was a precision machine with regard to the suspension? It's not and there will be quite a bit of variation in ride height based on temperature and how you stop the vehicle, etc. The suspension does NOT return consistently to the same spot all the time.
Did you measure it in EXACTLY the same parking location? That would affect it also.
Overall, a 1/2 inch either way doesn't mean anything. Now if it DOES go to 32, THEN you've got a wierd truck!