Between Up and FLT detent, there are no gradients or indents, to show amount of spoiler deflection desired. (unlike in the bus - think they have numbers?)
The flight spoiler accuator's are programed for a certain deflection to FLT detent. Now the Spoiler mixer is calibrated / Certified only AT THIS certain delection. Anything between is in the area of 'un-calibrated / un-certified'.
Try it - Initiating, or coming out of a turn, i.e. During aeleiron deflection, extending or retracting speedbrakes (passing slowly in-between the detents) will lead to a little dance of the control wheel. Now IF we had a mixer with a little shoddy rigging ?
The Auto-pilot roll rates are also calibrated / certified only at FLT detent and UP.
CaptainSandL has passed on the right 'advise from manufacturer' bit.
Cheers.