What Clay mentions is effectively a small amount of light that gets 'trapped' in the thickness of the film base, similar to how fiber optics work. Once it gets to the edge of the film, it dumps out onto the paper.

There's nother reason that the borders under rubylith can show up sometimes, in addition to this. A very thin coating on the paper has a much higher printing speed then a thick coating. This happens especially when using a brush to caot, as there are often 'wisps' of coating out at the edges that may be excessively thin. They will sensitize with much less UV exposure. If I see that, I double up the rubylith, and it normally takes care of the problem.