The stated times are an advised time. How long it will take something to defrost or cook will depend on the power of the microwave and the nature of the item being cooked/defrosted.
You could bung it in for a couple of minutes longer......
I suspect the amount of thawing required will depend on the temperature the food item was previously stored at. If it needs more time, give it more time.
It uses microwaves which can't get through the glass/mesh door. (And were the door seal to leak slightly I don't believe you'd notice it as underthawed food.)
Defrost time might be based on the average water content of foodstuff. Meat for example contains a lot more water per kilo than bread so takes longer to thaw, about twice as long in my experience.