Microwave Power settings
A microwave heats pretty unevenly — most of the heating goes into the outmost portion of the food, and even that isn't always even (e.g., bottom probably gets fairly little). That's not unique to a microwave; a pan mainly heats the bottom; an oven... 

Mostly, the lower power settings give more time for the heat to even out by inside the food. Depending on the food, you could run at a higher power setting and keep pulling it out to stir, rotate, etc. Often quicker, but much more work.