I noticed this a while ago.
I don't think that it's a very big deal. After all, the minirumble is for all robots that have a codesize of less than 1500 bytes, the microrumble is for all robots with a codesize less than 750 bytes, and the nanorumble is for all robots with a codesize less than 250 bytes. So, nanos and micros fit into the minirumble because, well, they are minis in the sense that they are all under 1500 bytes.
To say that minis and micros could could have their own exclusive rumbles would require different definitions of the weight classes. And, if you're going to all that trouble, you might consider more consistent brackets, like 250, 500, 1000, 2000.