The previous version of this package had a different version epoch to the current version but the upstream version did not go "backwards". For example, the previous package version was "1:1.0-1" and the current version is "2:2.0-1".
This was likely an accidental bump or addition of an epoch.
Epochs exist to cope with changes to the upstream version numbering scheme. Whilst they are a powerful tool, increasing or adding an epoch has many downsides including causing issues with versioned dependencies, being misleading to users and being aesthetically unappealing. Whilst they should be avoided, valid reasons to add or increment the epoch include:
- Upstream changed their versioning scheme in a way that makes the latest version lower than the previous one. - You need to permanently revert to a lower upstream version.
Temporary revertions (eg. after an NMU) should use not modify or introduce an epoch - please use the CURRENT+reallyFORMER until you can upload the latest version again.
If you are unsure whether you need to increase the epoch for a package, please consult the debian-devel mailing list.