You can't compare battery ratings in milliamp hours unless you know that the load voltage is the same for both.

Of course you can. If you read some datasheets you'll see that's how batteries are rated.

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.

Nexus 10 has a 22.75Wh battery, the HP Chromebook's is 30Wh.

Chromebook has a bigger battery than the N10, not a smaller one. Probably also a factor in why the Chromebook tips the scales at 2.3lbs vs. the N10's 1.3lbs.