I think the cancel has been enhanced with ink. Most of these Sz821A I have seen are fairly weak as the one I have is. I looked closely at my stamp's cancel and it appears to be done over the Mexico City cancel.
What all this means is, I don't know. It may have even been a cancel that originates in another district and incorrectly attributed to Mexico City. It may be a postage due mark for underpaid postage, or I have even heard of other things like a forward charge. It could have been used for city delivery. It is hard to tell, because I don't believe there is any real government regulations that covered this, or if they were, it has been lost in time.
Campeche has a number of fractional cancels and some have suggested they were used for rate changes. I don't believe this is a pre-cancel or it would have been done under the city cancel, leading us to think of it as an after thought, for whatever reason we don't know.