Height limit for infant seat...a guide or a rule?

Some of the older threads I found on height limits for infant seats note that these are just general guides and that the one-inch-of-shell rule trumps them. I'm wondering if this is true of all seats, though, since ours (Cybex Aton) has plastered all over it (and in the manual very prominently) that babies must be under the weight limit AND under the height limit AND with an inch of shell above their heads to safely use the seat.

I'm hoping this won't end up being an issue for us since if I follow DS' growth curve out, he shouldn't hit 30" until a year. (He's six months now.) But he was almost 27" at his six-month checkup, and has been growing over an inch a month since birth. (Maybe this is about to slow down...?) He still has lots of shell above his head and just moved to the second harness slots, so unless all three inches are torso, he will hit the height limit well before he hits the one-inch limit (and he's not even in the vicinity of the weight limit).

Are we okay to keep using the seat if he hits the height limit as long as he has more than an inch of shell, or should we consider it outgrown at that point? Thanks!

My infant seat ( graco Snugride) went up to 26 inches and 22 lbs. With my son we moved him to a convertible at 7 1/2 months because he reached the height limit.

I've always thought it was more arbitrary and really more for parents who didn't know the one inch rule.

Height limits are rather arbitrary. Take two kids who are both, say, 40") and stand them side-by-side. They're the same height. Now sit them next to each other and one looks taller, because one has a long torso and short legs, and the other has a short torso and long legs. Clearly they'll fit very differently in a car seat. Based on torso height, one might have already outgrown it and one might have a couple inches left.

As a tech, I can't tell you to ignore the manufacturer's stated height limit. However, most people are comfortable using seats beyond the stated height limit as long as the child still has sufficient shell over his/her head.