In this mad-cap world that is Atlassian plugins err...I mean add-ons...errr...whatever, the "Atlassian Universal Plugin Manager Plugin" is in fact not a plugin but an add-on.
You also may be forgiven for thinking that the "Atlassian Universal Plugin Manager - Plugin License Storage plugin" is a plugin but alas it is also an add-on. You can tell because it has an "Add-on key"
Something about not seeing the trees for the forest may be appropriate here!
It is just as well that Atlassian have the resources to manage this correctly or it could easily end up appearing to be amateurish.
When will it all end?