I'd like to preface this by saying I'm completely dyscalculate, so please explain this kindly but slowly 

I was born September 1990, my sister was born March 1992. 18 months between us and she was in the school year directly below me.
DD2 was born july 2024 so due to start school 2028. I am expecting DC3 in September 2025 so 14 months between them, with a school year in between them, (DC3 starts school in 2030).
How is this possible?? How can there be 14 months but a school year in between when there was 4 more months between me and my sister and she was in the year directly below me?
There couldn't be children in lower years older than a child in the year above them so what am I missing?