Naked-Edge or join problem

Hi dears
I have a problem with Naked-Edge or join.please see the video:

And the .3dm file
5.3dm (401.2 KB)


one of the surfaces is overlapping the other… trim the long one and they’ll join.

Thanks dear jeff for your answer it was very usefull for me but my question is :
when i have six surfaces+surface number 1 and join these seven surfaces(fig.1) i have Naked Edge (fig.2) but when i undo joining and fix the surface number 2 with these seven surface and then join these eight surfaces the surface number 2 can’t join to other seven surfaces_(fig.3) but the Naked Edge removed(fig.4)…why?