It is surprising isn't it Jalima?
Osteoporosis is a 'silent killer'. There are many factors that can predispose you to getting it, including genetics, age, gender and these we have no control over. But there are other lifestyle factors we can control such as taking exercise, reducing alcohol, getting sufficient calcium, and so on.
I'm fighting this condition successfully, without recourse to drugs like alendronic acid and strontium ranelate, both of which made me ill. So making sure I hit the recommended intake of 1200mg a day (on average) is really important to me.
It's worth using calculators like that Jalima to give you a rough and ready indication of how much calcium are in the foods you eat, and also to know which foods contain calcium (such as almonds) that might surprise you. Do you know that 1000mg is the normal daily recommendation, but if you are over 70 or have osteoporosis you need an extra 200mg?
Absent you are of course correct (though some foods do have added calcium, but that's a different issue) and yes, the milk in the yoghurt is that main calcium-rich ingredient. But I would still expect a company like M&S to be able to give an accurate numerical value for the amount of Ca per 100ml or per pot when such a question is relayed to their nutrition team, especially when others provide that information on their cartons. I suspect that many of M&S's tastier yoghurts are not particularly healthy and simply overloaded with ingredients to make them taste good and probably best avoided or treated as desserts.