One of the more interesting nuggets to come out of Millennial Media's latest monthly report (called S.M.A.R.T.) on smartphone ad-market analystics was this above chart showing average click-through rates in five (not four) app categories: games, social, entertainment, utility and navigation.

The chart above compares the smartphone platforms of Apple, Google's Android and Research in Motion's (RIM) BlackBerry.

A click-through rate of 2 percent for ad campaigns is considered "very successful." So which category blows the others out of the water? That's right: utility. All three smartphone platforms showed a high click-through rate for advertisments that ran in utility apps.

I'm a newbie to these ad analytics for mobile, but I'm very curious to peel back a few more layers of this onion, to see why smartphone users are more inclined to click on in-app ads in the utility apps, compared to the other categories.

So how about that Millennial and MobClix? What are your theories on what's going on with these utility apps and why are smartphone users more inclined to click on in-app advertising with them?

(Note: Millennial's monthly S.M.A.R.T. report was put together with statistics from MobClix. The above data are year-to-date figures.)