Cached information is updated once in a while, and updating those is triggered by normal pageloads. Each time a page is loaded a check is made if something extra has to be done, and if so, it does it.

So one time it might just load the page you are looking at, the next time it will also update the who's online island, and next time it updates an RSS feed etc.

That explains why the number of queries can differ each time you load the same page.

[Linked Image from siemons.org]