eBay

Why are my Flat rate envelopes 13% off when everybody is 15% off. I have several sites and only 1 (my biggest) is 13% off

My Flat Rate Envelope discount on one site is 15% off and another is 13% off. For 22 years all percentages were the same, I checked with other people and everybody else is 15% off. Nobody from ebay has an answer. Has anybody else have this problem.