[TECK]

The value 0KB is displayed when PHP is not compiled with the option "--enable-memory-limit". It's just an indicator for you to know that the memory limit is disabled in PHP.
Otherwise, you will see values like 4,236.85KB, instead of 0KB.
All you have to do is contact your host and ask them to enable the option.

In vBulletin, a datastore is used to store into memory useful variables, templates, etc.
That system avoids calling the database each time you process some global info, increasing the script performance.
If you create a modification or a separate file and forget the cache your new templates (using the array variables defined into default vBulletin files), then each uncached template will execute a query instead of being stored into server memory.

Let's take an example. You install a new hack.
The hacker who created the hack forgot to cache one template. The result?
One extra query performed for nothing in each vBulletin page. The forum display would have 14 queries instead of 13 if you missed to cache 1 template, 15 queries if you missed to cache 2 templates and so on...
The microstats will show you always if there are any "forgotten" templates on your page, in order to avoid performing not needed queries. It will display the number of un-cached templates as well their names. The un-cached templates will be highlighted in light-blue, like in the first screenshot.
Cool, huh?

I hope this helps clearing your questions.
If I missed or forgot something, please let me know.