The reason is you have your system in "Stack" mode
there is a button right below the G3Port5 Tab.(in GS3Orch)

It looks like 2 dark grey rectagles (with Green "LED'S" and one lighter grey rectangle.
When pushed in ther will be a lighter border around it.
The button next to it, Audition Mode (looks like paper with the upper right corner folded) is the one to push if you want to
have new samples replace older ones on the same midi channel.