Well, without knowing which aircraft its hard to guess....
I've seen (again in the military context) a distinction being made between the initial turn rate (number of degrees of heading change in first second OR time to achieve first XX degrees) against the rate achievable once established in a turn. In that case the initial turn rate is reduced below the sustained rate by the need to roll into the turn first.
But whether the "instantaneous" you are seeing is what I'd think of as "initial"...?
Perhaps quote the numbers you have and the conditions and aircraft they apply to and someone might be able to reverse engineer the specific definition?