Overlap support for three different releases for a month (13 months support)
For many maintainers, it's convenient/only realistic to skip one sdk/runtime release on Flathub. For example update from 43 to 45.
Currently, it's almost impossible to hit this upgrade path without shipping an outdated runtime for a while. Therefore, it would make sense to support each release for 13 months instead of 12. This would allow for a reasonable upgrade timeframe on Flathub.
Examples: With Document Scanner and Videos we even have two Core apps on Flathub that are still on 43.