I think it's pretty universally known amongst TW players that Rome: TW (which doesn't appear here) is more popular than Empire. The reason is - most Rome players don't use a steam version. I wonder how much off channeling affects things. For example, I have over 100 hours in the Binding of Isaac, but my Steam version has 0 hours of record, because I play the DRM free version. There are many more like this, in my case. So the % unplayed is probably even lower than the revision, because items purchased DRM free with a steam key addon (The Humble Bundle, most notably, and several Kickstarter games) are only logging hours for those connected to the DRM engine. This doesn't matter for the purposes of this reporting, since the focus is only on steam, but the takeaway is: Games released after March 2009, that were only released to steam, are likely dead-on accurate (ex: Skyrim) with the exception of offline play mode. If there are other release channels, it only reflects a slice. The biggest slice in most cases, but not all. Rome: TW I already mentioned, but M&B:Warband has a rather large following that steam doesn't even begin to scrape the surface of, and Arma2 (aka DayZ Mod). Those are just off the top of my head.
Valve has always been rather cagey about this sort of thing, and I'm sure they don't like the fact that data they may very well consider trade secrets seems to be leaking at an alarming rate. Flatland manual dropped a few years ago; Greenlighters leaking all sorts of random bits because they don't know they shouldn't be doing that; and lastly this: the website and interface structure leaking something close to actual sales data within a 10% margin of error. I do have to wonder if Valve will 'plug the security hole' or if they have solidified their position as market leader to the point where they've stopped caring, and let the chips fall as they may. Still, the ability to render corporate secrets by aggregating relatively benign bits of data from the published content on their website is a fascinating exercise.