So we'd like to know why this rule is. Does anyone of you know?
We've found out that the Swedish seats are designed very differently from the US seats, which is why they can be used rear-facing until ears at top of shell (which is also how they can recommend that kids rf at least until 4 or 5, preferably 5, because nearly all kids can, in the erf seats). So we're sort of hoping to work out how whatever the rationale is for the rule applies to the Swedish seats.
We're choosing between keeping our Brio Zento (where DD has well over 5 inches left until her ears are at the top of the shell, but only just over 1 inch until her shoulders are 1 inch above the top slots), or selling it and buying a Radian (where DD might have 2 inches left until outgrown by any of the rules).
I'm in the UK and I've never seen the rule of shoulders not being more than 1 inch over the top slots. We have 2 britax seats, both of which say the seat can be used till the child ears are at the top of the hard plastic shell. Though I suppose it would be an unusual child who's shoulders were more than a iinch above the slots but still had their ears below the shell.
You've got me interested now so I'll be waiting to see what others say.