Why are there no iPhone 9 and Windows 9 versions?

The number is just an identifier. None of the versions reflected the actual release order:

[First design spec group]

The 1st generation was just called iPhone.

The 2nd generation was called iPhone 3G

The 3rd generation was called iPhone 3GS

[Second design spec group]

The 4th generation was called iPhone 4

The 5th generation was called iPhone 4s

[Third design spec group]

The 6th generation was called iPhone 5

The 7th generation was called iPhone 5c and 5s

[Fourth design spec group]

The 8th generation was called iPhone 6 and 6 plus

The 9th generation was called iPhone 6s and 6 plus along with first legacy model SE* (*basato sul terzo gruppo di design)

La decima generazione si chiamava iPhone 7 e 7 plus

L'undicesima generazione si chiamava iPhone 8 e 8 plus E iPhone X**

[quinto gruppo di design** e sesto gruppo di design