jewish girl – You have a point when it comes to people who in principle do differently inside the house than outside. But many just do it practically, i.e. in truth they have no problem with c.s. but since they want people to eat in their house they only have c.y. at home. This kind of thing is fairly common in Israel with regard to different hashgachos; though many people eat, say, Rabbanut, they will only bring Badatz into their house so that more people will feel comfortable eating by them.
Another thing is that even among those who forbid c.s. there are those who say it’s only a chumra, not a halacha. In your own home you may want to keep this chumra, but when it is at the expense of another person’s honor it might not be the right thing, hence the “we only eat it at our grandparent’s house” rule.
On a side note, it is always important to know what is halacha and what is only chumra, especially for this kind of situation.