Choosing to share or not: 02 (A recipe)

A then-student (French, with her mother having half English blood) told me something very interesting at least two decades back.

    Her mother, when asked for the recipe of a dish that the guest(s) had found most delicious, would hand it over (as a handwritten copy in those days) -- but with some ingredient(s) missing, so that it wouldn't turn out to be the same flavour (or as tasty).

    Is this competitiveness on a cultural basis (i.e., is it a common French practice) or on a gender basis?

    Or just personal: my student's mother was just protecting her own interests (which is very wise practice -- not to be over-generous in sharing)?

    I know what reader Valerio will say: not on the first two counts -- too much of a generalisation!