I'm sorry, but this is the wrong manual... Judging by the image of Cooper on google, he was in a keyfit30. It goes to 30inches tall and 30lbs.
That said, if he were still under 30" tall, he would have fallen off the chart by about eighteen months.
I do take issue with the assumption he simply didn't fit in a seat because a mannequin of equal size was too large. Mannequins don't slouch or settle in like a child does, and every child is shaped different. I'd have a hard time buying that the mannequin was of equal proportion all over, myself.
I do believe that he probably didn't fit the seat and that it was used wrong.
Given that 90% of seats are used wrong, I think it's better to assume the 10% that get it right are extra diligent rather than the other 90% careless or neglectful. The concept of car seats saving lives is very abstract. Look how many posters have argued that rear facing seats are ridiculous past a year, or look uncomfy, or whatever, despite the fact that they are known to be four times safer. So many go by the law rather than the recommendations because a ticket or fine is an easier understood risk than injury to their child. After all, we all rode without these modern seats, and we survived.
And all that said, I think these two were clearly meant for each other and both guilty as sin