Mold on cakes? Full question?

It may be coincidence. The cake mix might have contained some mold spores before opening, or some spores in the air may have landed on it while being mixed. You cannot really tell. I don't really see any preservatives in the cake mix, although I am not an expert on them. Your homemade cake contains some of the same ingredients as the mix. but maybe was not exposed to the same mold spores. Also, perhaps the cake mix cake was more moist, which would encourage mold growth. There is no way to tell exactly, and next time it might be different. This is why experiments must be done a number of times to get reliable results.