I doubt it was because you didn't 'prep it'.
The key to consistency with any powder measure is consistency in the way you mount & use it.
If it wiggles or shakes around on the mount each throw, it will throw different charge weights.
If you slam the handle down one time, and ease it down the next, it will throw different charge weights.
You Must operate it the same way every throw.
You must run about 10 charges thru it after filling or re-filling the hopper, or changing the setting before it settles down.
rc