Why is the Plate and beam moment sign does not change with pressure load direction?

Good Day,

I am just new to Staad Pro so Kindly explain to me in very preliminary way,

This is simple Slab of 350mm thickness Supported by beam of 350x350mm on four sides supported by pinned supports an uniform pressure load of 100kN/m2 is applied on the slab in both +ve and -ve Dir of Local Z axis of the slab separately as illustrated in the pictures but the bending moment results of slab and beam are not changing signs why? and In both cases the slabs are sagging correct me if i am.

Thanks in Advance