Now THAT is weird. I know a little about .JPG format but I didn't think it had text elements. It is essentially a PICTURE compression format, not a text compression item, so text rendering shouldn't even enter into the picture. (Sorry for the pun... actually, no I'm not.)
JPEG encoding works on 8x8 pixel blocks, so there is a possible issue with image losses looking a bit "pixelated". If the characters happen to overlap multiple blocks, they could easily have different transform results based on the color compression method being used, but that would cause color inconsistency, not necessarily blockiness or smoothness in different cases.