Same here. WHen I notice one day my kid is 40 lbs, I'd stop using it. Most people have no clue what their kids weigh unless they are at the doctor nor do they weigh them at home. I can't think of one friend who knows what their kid weighs and the only parents at car seat checks who know are those who just had a doctor appointment.
In fact I have no clue what DD2 weighs. She got weighed at her 2 year well child check and not since. She'll get weighed again next month when she gets a flu shot. I really just don't care. One day she'll get weighed and she'll be 35 pounds and if I had a 35 lb seat I'd stop using it then. If she's over by a pound, I'm not gonna sweat it. Just turn her. I have no intention on weighing my child daily, let alone with a full belly, clothes, jacket, shoes etc each time we leave the house! LOL And I'm a tech! I can't imagine the average parent knowing what all their kids weigh day to day or weighing frequently. I only know DD1 for obvious reasons.
If a car seat magically failed at 1,2, or even 3 pounds over you'd hear about a whole lot more serious injuries from seats failing from children too heavy for them. But you just don't. And you can bet a lot of parents are over the weight limits especially in infant seats. But there just isn't a whole lot out there that this is happening frequently. If the seat failed at 41.0 pounds, it probably wouldn't be rated to 40
I never would intentionally use a seat over the limits, but if it happened cause I didn't weigh my kid that week, I wouldn't sweat it(especially when the identical seat is rated 7 pounds higher a 4 hour drive away from me)
But I'm also the person who isn't analyzing harness slots daily as well with a pencil in the slot taking pics asking if it should be moved up
When I notice one day she looks a little over, I'll move it up. Not gonna sweat it. In fact she was in DH's car this week cause mine was in the shop and I noticed she was over the slot by about 1/2". I'm never in that car with her. I hadn't noticed she was even close prior. It didn't even concern me she was over a little bit. I just moved it on up. I just don't think seats are going to fail and injure my child if I'm a tiny bit over the height/weight limits. I'm with Splash that there is a built in cushion since 80% of America is using seats incorrectly and they know that.