One common theme I've noticed among saddles is that when I sit on them, they bow inwards at the middle where there's no support from the rails.
In order to compensate for this, I find myself tilting my saddle back a bit farther than I'd like it to be, so that my weight stays on the sit-bones. If I don't tilt the saddle back, I end up sliding forwards.
I once had a really horrible BSO saddle that suffered heavily from this effect. When I sat on it, it basically became a banana. I ended up jamming a block of wood between the rails and the bottom of the seat, and it fixed the issue. I've since upgraded to a better saddle (Velo), but it still does a bit of this.
Is this supposed to happen?
I ended up solving this buy moving my saddle back. It kept me from sliding forwards and allowed me to nose the saddle down to level.