My guess is that the protein powders are made from brown rice or even rice bran and rice germ whereas most rice in the US is eaten as white rice (i.e., rice without bran or germ). I'm guessing that makes a difference.

Also, if you cook rice as if it were pasta (i.e., in much more water than needed, which is then discarded at the end of cooking) as recommended by the USDA, the amount of arsenic is cut in half. The manufacturers of protein powders probably don't bother to process the rice this way (whereas I do when I cook rice).