this is a behaviour I saw several times.
You install a plugin from Spartacus and deaktivate (delete) it some time in the future.
In case you want to install it again Serendipty sees the plugin on disk and will not get it freshly from Spartacus, it does not even recognize there is a newer version on Spartacus.
Suggestion 1: In case you install a plugin, which is already on disk, look on Spartacus to download the fresh version.
Suggestion 2: If a plugin is deleted, delete it from disk as well.
In my opinion either way, Suggestion 1 or Suggestion 2 will erase the issue. I prefer Suggestion 1 over Suggestion 2 because this would affect bundled plugins as well.