It’s hard to design a microwave oven in which the intensity of the microwaves is completely uniform throughout the entire volume of the box so that food in all locations will be subjected to the same heating power.
Moreover, any food in the oven is sucking up microwaves and upsetting whatever uniformity there might otherwise be. You can buy an inexpensive microwave-sensitive gadget in a kitchenware store, put it in various parts of the oven, and see that it registers different intensities at different locations.
The solution is to keep the food moving, so that it averages out non-uniformities in microwave intensity.
Most ovens these days have an automatic turntable, but if yours doesn’t, many recipes and defrosting instructions on frozen foods will remind you to rotate the food halfway through the heating time.