The generator that creates the energy to heat the food is not uniform in distribution. Its a wave of energy that bounces around inside the microwave box. The waves bounce around inside the box and hit your food. If you could see the the waves bouncing inside the microwave you would find that some waves overlap each other and some waves cancel each other out. This effectively creates cold and hot spots inside your microwave. Many microwaves today overcome this issue by putting a spinning blade at the top to reflect the microwaves around to different areas, or a spinning plate on the bottom to move your food around.
This picture show how the interior of a microwave without spinning and with spinning the plate. As you can see the center seems to be an area that doesnt really get the benefits of reflected rays and is an effective cool spot. You can replicate this experiment by taking out your plate and lining the bottom of your microwave with marshmallows (google this, dont try at home). You will see the areas that are hot and cold.
Youtube video https://www.youtube.com/watch?v=kp33ZprO0Ck
Thank you for your reply, I was planning on asking a follow up question like if any microwaves can evenly distribute their waves but then you went and answered it in the second half of the response, well done!
The short answer is unlike the hot air in a conventional oven, microwaves are not evenly distributed. And most microwaves have spinning platters inside. The closer to the center, the less the object moves and the farther to the side, the farther it will travel as it spins. This movement cancels hot spots and dead spots and provides more consistent cooking.
Imagine that a microwave oven cooks with light instead of microwaves. The way it’s designed is that the bulb sits away from the cooking area and a little reflective tunnel connects the bulb to the cooking area by reflecting the light back and forth until it reaches the inside of the cooking area, where the light can then reflect off of the shiny surfaces inside the microwave.
With the light coming from a little tunnel off to the side, and the inside of the microwave being very reflective (to microwaves if not light) you can see how difficult it would be to get a super even distribution of light. There will inevitably be hot spots and dead spots which is why many ovens contain rotating platters.
Placing something off center allows it to move around because the farther it sits from the center of the spinning platter, the larger the distance it covers. This means it will cook more evenly and consistently as any hot or cold spots will tend to average out.