Batteries are rated in milliamp hours at a given voltage. You can't directly compare ratings in milliamp hours without accounting for the voltage. Watt-hour ratings, in contrast, bake assumptions about the average battery voltage into the rating, and thus can be compared directly even between batteries having different voltages.

In this case, it's common sense. There is no way a Nexus 10 has a battery four times as large as an 11.6" notebook.