So the plot thickens:
I went ahead and picked up an adjustable measure to remove any further doubt over the accuracy of my measure spout. It's a pistol measure with charges from 5 to 45.
So first of all, my 30 grain spout is still showing to be accurate. The amount matches between the spout, the adjustable measure, and by weight. HOWEVER this time when I actually put powder into the chamber it didn't fill it to the top like it did when I tested the other night (I haven't tried loading a ball yet).
Also, while doing testing, that little gate/door/whatever on the inside of the mouth of the powder flask FELL OFF and dropped down into the flask (yay, now I need to see if I can actually open the flask all the way to get it back).
So now I'm wondering if maybe that door wasn't seated right, so when I went to fill the chamber it MEASURED accurately from the spout, but didn't seal so when I actually went to load the powder more came out when I upended the flask.