netmeg - 7:53 pm on Jul 6, 2011 (gmt 0)
It means that they're probably on smartphones and tablets, and some of them can see your normal AdSense - but some of them may not. For example, my current BlackBerry doesn't see regular ads, but my old one did, and some versions of Safari (iPad, iPhone - and in fact my current BB displays a Safari user agent for some reason) block ads as well.
So you might be earning revenue, but you also might not be earning as much revenue as you would be if you served up mobile ads.
I will tell you this, having just come off the peak season for one of my sites. Mobile ad earnings were four times what they were last year, and while the CTR was lower than for the regular ads, the EPC for mobile ads was WAY higher. Like, three or four times as much as regular EPC, over mid three figures worth of mobile clicks/day type numbers. YMMV.