I saw a notice for an Open call on the board at Equity and on Casting Call, but in only some of the trade papers. Why is that?

The trade newspapers are not affiliated with Equity; they are private businesses, and each has its own advertising policy. As a courtesy to our Members, some trades print notices for required Equity auditions at no cost to the union. Some notices of non-required auditions may appear in only one of the trades because the producer chooses to pay to advertise in that publication only.