String coupling.

[Larry J. Messerly]

snippet

 No, a  single string won't have as much active affect on the bridge as
three, but  even with two strings muted off, they are still coupled to
the active string through the bridge and acting as dampers to the active
string's  motion.

Ron,
I admit that I am way out of my league here but it seems that the muted
strings would impede the open strings force on the bridge.  Making the
coupling even more pronounced when they were opened up to vibrate.

Also, there is a lot of very complex motion going on at the bridge, up,
down, top, bottom, strange and charmed motions.  However, I cannot
conceive of the bridge moving left to right nearly as much as up and down
(on a grand).

Larry Messerly