Adjustable powder (volume) measures are notoriously inaccurate. I have 4 that don't agree within 15%. The same goes for spouts.
However, the bottom line is that it doesn't really matter - one of the best kept secrets in black powder shooting. What DOES matter is that you be consistent in making up your loads. So, pick one adjustable measure and stick with it. Adjust the flask spouts by trimming them to work with that adjustable measure.
And don't go getting all hinky about whether you're measuring 21.1 grains or 20.9 grains, for Pete's sake. 5 grains accuracy is plenty close enough. Unless you're shooting national bench rest competition, in which case there's already no hope for you - sleepless nights are ahead.