Alright, I figured it out guys. TL;DR: I'm kind of an idiot.
PaulRB was mostly right, but to put it more simply: I'm generating this circle by stepping through values of X, one at a time. At x=0, y=radius of the circle, the Y values change very slowly, hence the smoothness. At x=radius, y=0, I'm still only stepping X by one value at a time, but Y changes more per value of X, giving it a choppier look.
The question I asked was essentially flawed from the start. Thanks everyone for the responses.