Has license file behavior changed when running multiple sessions?

I may be wrong in my understanding of how the Autodesk license manager is supposed to operate but it doesn't seem to be acting the way I expect. My office has 15 perpetual licenses on maintenance (I believe that's the correct terminology). I thought that meant, on a given machine, the user could open up a session of let's say Revit in the 2018, 2017, 2016, and 2015 releases all at the same time and still effectively only be using one license. One license would be used for each version but there would still be 14 licenses available.

I was hit with a license limit error yesterday and when I opened the LMTools to make sure everything was working this was not what I was seeing. LMTools was telling me that all 15 of my perpetual licenses were in use while users had some combination of sessions open. So there would be 9 sessions of Revit 2018 open and 6 sessions of Revit 2017 open and the license manager wouldn't grant access to a tenth Revit 2018 session.

Did I misunderstand how the licenses are supposed to work or is my license file not acting properly?

Multiple sessions with a single version should use a single license. Evidently the changes between Revit versions show up a separate programs, each version pulling a new license?

Different versions will pull a license for each, while the same version will only use one. Same for the Suite licenses and products in that Suite, so if you have Revit 2017 and Navisworks Manage 2017 running together it should only be one (ultimate) Suite license; if you have Revit 2016 and Manage 2017 that would be two licenses.

What @dgorsman said - It's all spelled out clearly, here.