The IO standard is written in a generic VHDL. When opening post-translate PACE, the IO standard and slew rate are taken into account, but the drive strength is not.
In Floorplan Editor, the drive strength cannot be added from NGD. This IO Constraint must be read from the PAD. All other IO pad constraints are read from the NGD because they are on the pad.
This issue is fixed in ISE 10.1sp1.