Because you have 3 UR dark magician and only 1-2 copies for other SR?
Think from another perspective. Assume you have 3 UR and only 1 copy of all SR. It means you have a TOTAL of 3 UR and 4+SR. Now, which rarity has higher drop rate?
4URs and the SRs was from completing the missions (like battle him 3 times, 5 times...) 1 each, still 0 Gaia, all i got from the drop are Rares, Normal and URs.
(sorry my english is bad)