I don't remember the exact dates that each system has been acquired by Wyndham, but since Wyndham holds the reins over all 3, they can do what they want.
When Worldmark first got taken over they allowed all of Wyndham to book directly into Worldmark No fees, no club pass, VIP discounts applied. But WM owners sued and got an agreement that Wyndham can't book into WM and vice versa. The result was that some locations did an inventory swap, and that is why some resorts are dual branded both Club Wyndham and Worldmark. But later they introduced Club Pass which allows for a booking in Wyndham points for a WM for a fee, and vice versa for WM. But not all inventory is available, and there is a points balancing that must go on in the background.
Similar with Shell that Wyndham acquired. They put inventory into both systems. They have just moved it out of WM and put it in Wyn. Suppose that you could get it through club pass if you have it.
But liked my direct access to WM via wyndham. But what wyndham can give, wyndham can take away.
So now I own both wyndham and WM, it gives to 2 paths to the inventory I want.