Most likely because you have a dilithium bonus pool active from completing the Ferengi Tour Of Duty 10/10 in the Admiralty system. Which will give you 50% extra dilithium ore on anything that gives dilithium as a reward, until the bonus pool is used up. Perhaps your other characters don't have this bonus pool active currently.
Although I've not tested this bit, I would imagine the same would happen during a bonus dilithium event too. If so, all your characters should get the extra for trading in their marks.
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Although I've not tested this bit, I would imagine the same would happen during a bonus dilithium event too. If so, all your characters should get the extra for trading in their marks.