The Kindle's high battery life when you are reading comes from two things.
Firstly, the Screen only uses power when you change the content of the page. This means that no matter how long you read for, only a finite amount of power will be used when you turn the page.
Secondly, the
CPU in the Kindle will be doing very little (and consequently using a small amount of power) when you are just reading.
When in a game or other active content, the screen will be refreshing more frequently and the CPU will be working much harder. Both of these would result in a noticeable drop in battery life.