There's probably lots of factors in this issue. Cream is rather different through out the year, winter cream is fattier than summer cream on average. Cartons don't tell you what you get of course, but the time of the year should be sufficient.
There has been a trend to less fatty creams anyway, meaning you're getting more protein in the cream. Protien is what curdles. Pasteurization methods could also play some tricks. Whether they used the high heat or low heat pasteurizaion. UHT Cream is even more different in flavor and texture, but I bet it wouldn't curdle much.
Again, Lemons vary in acidity too depending on growing conditions.
There are so many variables outside of your control there is no simple answer.
Phil