The 1.7.2 version has a bug in its Software Update mechanism when running on Snow Leopard (it's fine in all other versions of the OS) where the update will actually fail. Maybe that's the 'fast screen' you're talking about?
I'm not sure what the 'newer' version might be, but the Finder is simply reporting on the *date & time* of the copy in the Applications folder, not the actual version number. It could be a 1.8.2 beta version, perhaps?
In any case, there is no reason not to drag 1.8.1 into the Application folder and overwrite what's there already. Version 1.8.1 is the current release. There is nothing to stop you upgrading!