I have been saying this for a long time that they have divided affiliates into different groups, they shave sales from one group , then give them good sales and start shaving from the other group and so on.
About checking by buying premium yourself, I don't think it will work, because there are different methods used by RG, maybe they will count one month sales on that day but not 3 month sales or vice versa, maybe they will not count sales from US but will count sales from other countries on that day, and so on, there can be many variations, so I don't think you will get an exact answer by doing this.