There are not 8 major versions between iOS 18 and iOS 26. Apple skipped the monotonously increasing version numbering system since iOS 1 during WDDC 2025 to adopt a year suffix based versioning system.
Edit: Oop, I misread! Right, yes, the change up was arguably not entirely boring. Some people were excited at least.
Originally: To be the annoying pedant, version numbers did still monotonically increase, even with the gap, because each version is >= to the last. The mono means a single direction, not a step size of one.
iOS 17, then iOS 18, then iOS 26, then iOS 27.
You're not the only party confused.