On the Unity Webplayer page, what's the difference between plugin and engine version?

I’m currently debugging on several machines, when checking version numbers I see varying results on Unity Real-Time Development Platform | 3D, 2D, VR & AR Engine

Sometimes the plugin is 3.5 and the engine 4.0, and sometimes the other way around. What’s the difference between these two?

The plugin is the ActiveX control or NSAPI DLL version. The player is the runtime code, so the equivalent of the standalone player executable. You can use pretty much any plugin. The player that is shown/used is the one that matches the version of the editor used to create the content.