`priority` is an attribute of a `Source`, which is delegated through `ModuleRelease` instances for the sake of comparison (sorting).  By default, the `InstalledModules` source has a priority of 10 (making it the most preferable source, so that already installed versions of modules are selected in preference to modules from e.g. the Forge).  Since we are specifically looking to upgrade this module, we don't want the installed version of *this* module to be chosen in preference to those with higher versions.

Having said all that, defining a singleton method is obviously a suboptimal solution, and we should probably refactor this eventually.  In the meantime, I will leave a comment explaining the behaviour.