The turntable is there because the microwaves bouncing around inside the unit don’t do so completely evenly; there will be spots that don’t get heated as much. Moving the food around inside the unit makes it so one of those cold spots isn’t consistently in one place in the food.

So with that as a context, if a cold spot is at or near the center of the turntable, it’s not going to get moved around in the food when it rotates. So better to put the food away from the center.