Even gilt brass medals have this orange peel texture, too:
When this observation was brought up with the moderator of the Chinese brass medal QQ group, Li Haoyu, he stated that the bumpy surface (orange peel) resulted from the annealing process, as brass blanks were subject to extremely high temperatures. He had learned it from insiders of Shanghai Mint. This revelation is similar to the discussions in this thread: http://forums.collectors.com/messageview.cfm?catid=26&threadid=899301.
Interestingly, the "orange peel" texture can be used as a characteristic to tell brass coins and medals from this period of time from later fakes.