Most of the value/cheaper tinned food that I buy, don't have ring pulls, so that could be a factor.
Likewise, my DD (with more disposable income than me), buys the microwaveable pots of beans and fresh/frozen veg.
I also use dry cat food.
If you've been in her life since she was two and she only hasn't been to visit in a year or so, Why hasn't she been shown this, since around 8-10 years old?
Where I've lived, in Liverpool, the children seem to be given a lot more responsibility, younger than those on MN and are joining in with cooking, looking after younger siblings, going to local shops, from about eight, but because of shopping habits, may not come across ring pulls on tins.
I have to lift mine with a spoon first.
Did you not think to ask your DH, why this was?