Hi alex.
David Davies has explained this behaviour very well in his "Handling the big jets".
That's not a calibration thing, as you've heard, but the very design of the system.
There are two kind of spoilers: defferential and not.
On the first type, the one we have I presume, when the spoilers are half extended, if you enter a (right) turn the panels on the (right) wing will extend more, and the panels on the (left) wing will retract, thus causing a high roll rate.
I've experienced this as well flying manually, the rate was quite high.
You can try again anytime.
On the NON differential type, the spoilers on the (left) wing will stay where they are, and you won't notice any increase in roll rate.
This could be used to your advantage while flying an approach with degraded flight controls, with one channel blocked and one of the pilots operating only one aileron, for example.
You could keep the spoilers extended a little bit to increase a poor roll rate capability.
Cheers, LEM