question about moving car seats

[Rowlers]

By that I mean moving from baby car seat to the next stage up.
DS is 9 months but quite light - 18lb 9 weighed today.
So I think I'm supposed to wait until he's 20lb
Question is, why?
Not that I'm questioning it, just would like to understand the safety ishoos.
Anyone know?
I did ask hv today but no idea what she was talking about.

They need to be a certain weight for the seat to anchor them correctly and be able to withstand the force of being thrown forwards in the event of a sudden stop(bearing in mind the head is still the heaviest part). There is research to suggest that rear facing is better until at least 4 due to the potential effect of such forces on small bodies.

Thanks!