[cyphar]

Rice cookers are also covered vessels (though they have a small vent), so even if you are cooking rice on the stove evaporation would have a similar impact.

One way to check might be to try to cook the same mass of water as though it were rice and see how much water you've lost through evaporation. 250mL (1 cup) of water really isn't that much liquid to lose in ~20 minutes during cooking -- if you've cooked soups before you may recall that uncovered soups will lose far more than 1 cup of liquid in ~20 minutes.