Definitions of project management roles aren't strict. I know people who are a few times more than others and earns at least a couple of times more despite all of them have the same title - Project Manager.
Also I'd be far from assuming that a specific role name means PM is dealing with specific method. You will find Project Managers or Senior Project Managers who don't work with any specific approach, as well as those using formal methods and those basing rather on agile techniques. Business card tells you neither of those.
Senior Project Manager doesn't have to automatically manage other Project Managers. I'd say that it would be a role more of a Programme Manager than Senior Project Manager.
Specific area where a Project Managers works (software development/IT) tells you more about typo of project that they're dealing with than about their seniority, experience, etc.
If I had to draw any conclusions form salary differences I'd say that generally senior roles earn more than basic ones (which is obvious) and that there differences in salaries of the same role between different industries (which is also obvious; look at rates of project manager in construction). I wouldn't go much further.
By the way since in real life definitions of roles are rather vague don't treat research basing on that as an oracle.
And one more thing: if we discuss hourly rates they would differ vastly depending on a company, world region, etc, which is another reason not to treat these result so seriously.