Boundary surface end is not on boundary

I am working on splitting a complicated corebox and used  a ruled surface  and a boundary surface on a corner but the surfaces are generating off the corner.

In this picture the corner of the solid was picked for both the edge of the ruled surface and the edge of the boundary surface and both ended up off the corner buy about .030"

These surfaces are not to split this body but one made from the reverse of this body and another.