Diets.....weird

In particular, why is my BIL/SIL weird! yes, I'm being cheeky and talking about Andys brother and his wife, who are both lovely, but I txt'd them to say, what food would you like this weekend? and they texted back to say, don't worry we're bringing our own!! I replied, 'own? you can't go bringing your own, I can cook slimming world, its ok folkes' They replied don't worry we're doing really well and will bring our own, WTF??? how very odd?
Anyway I'm doing fahitas for us, which fit in nicely with slimming world, so if they only bring a lettuce leaf, I shall sit there and gloat! vanity, schmanity says I.