The extension is the same for 5.0 and 5.1, hence the version '5.0'. What the extension does is look at a registry key (which is added/removed by the installer) where the designer is located and load that designer. It checks from 5.9 to 5.0 and loads the first designer key found, so always the latest version. If you look at the Help -> About dialog it will show v5.1. The VSIX is more of a wrapper around the designer, as for normal vsix extensions the actual extension has to be placed in the vsix extension folder itself, but as we also offer a standalone installer it would mean we either have to copy the files twice, or use some trick to make the vsix load the installer from the actual installation folder. Which is what we do
The problem is that if we want to version it, we need 2 extensions as building one on the same box for v5.0 and v5.1 doesn't work, or at least it gives a lot of problems (as the extension compiler also installs the one it compiles). As the functionality is exactly the same we keep it as-is.
Does this give problems for you?