I don't have hands on it, but I will not be surprised if supercoiled DNA migrates at different distances according to some inner topological conformation (i.e., more or less coiled AND/OR more or less nicked). Similarly, this picture highlights >8 conformations. What is run in the gel is circularized phage DNA with some degree of knotting due to the circularization (phage DNA is otherwise linear).
Figure from: Arsuaga et al. 2005, PNAS 102 (26) 9165-9169 (free on PubMed Central)
Another theoretical possibility is that two or more plasmids of different sizes were grown in the same maxiprep because of some contamination. But in this case, you will spot the problem also in the digestion, so it does not seem to be your case.