Why don't gantry systems "twist"

I've been looking at photos of gantry based routers, etc. and I cannot see why the gantry doesnt twist (i.e. varied travel on one side as opposed to the other).. Are they generally "ganged" using timing belts, or is there a motor on each side driving the gantry in lock step, etc?

The designs generally don't look stiff enough to mechanicly resist twisting..

- jim