From [0]:
"Because of dependency on architecture and system changes to any current version of Apple operating systems (for example, macOS 26, iOS 26, and so on), not all known security issues are addressed in previous versions (for example, macOS 15, iOS 18, and so on)."
[0] https://support.apple.com/en-ca/guide/deployment/depc4c80847...
[1] https://arstechnica.com/gadgets/2022/10/apple-clarifies-secu...
The little blurb I quoted is sort of an excuse for some other updates.
People with a more modern iPhone or iPad who chose to avoid updating to iOS/iPadOS 26 are given only an option to update to 26.3.
Edit: 26 macOS and iOS/iPadOS suck, I won't use them.