[mark_in_manc]

I suppose if there were prime relationships between the periods of the oscillators - that is, their frequencies were all prime multiples of some fundamental bass 'sub-harmonic' - then they'd all come in-phase only at that (long) period, but would look incoherent elsewhere.

(Perhaps interestingly, in acoustics the ear can spot such a 'missing fundamental', and this forms part of the academic argument for how pitch perception works).