I figured when the hunter was telling his story while pointing to his drawings, the reason for the multi-heads would become clear without flickering.  So this recent observation that the flickering flame does that leads to the assumption the cave people knew that. But whether the drawings were made with the flickering in mind we'll never know.
That said, it's a pretty cool observation considering people haven't looked at these drawings by flame light in a very long time. LEDs don't flicker much. 

I wonder if they've checked this out with other caves? I guess it would work best with heavily incised images rather than just "painted".