What Tam said ^^^ Sometimes they want to leave the Nic the same as well and mix the same PG/VG and nic as the pre made to cut it. Why? the flavor may be to strong and they would like to cut it back or just make a 30ml bottle last a bit longer. Some will do this and ad extra flavoring to change it up. The combos are, in a way, only limited to your imagination and of course, the math.