Upgrades can only be done between consecutive major versions (you can’t skip one). Owncloud and Nextcloud versions are related, but the numbering is a bit different:
OC 9.0 ↔ NC 9
OC 9.1 ↔ NC 10
You could probably upgrade from OC 9.1 to NC 11 but not from OC 9.1 to NC 12. The updater app “knows” this and only provides allowed update paths. There are different release channels which define the max. version you can upgrade to (channels are beta, stable and production). beta always has the newest stuff (and can be unstable, therefore only to be used for testing setups), … production is the most conservative update channel which only uses well tested versions.
It’s currently on discussion when a version is considered stable enough to be stable or production: