a 7 pixel gap is a little high. I get away with 4 pixels and I think 3 pixels work also.
the simple answer is: I don't know but it's necessary.
also the Maximum frame graphic height needs to be the same height as the caption frame height.
and the OS caption height (extra settings) should equal that height as well.
I usually take the caption frame, resize the height of the individual sections -4 pixels and add a blank/transparent 4 pixel border to the top. It can take some planning not to disrupt your design intentions.