So sorry for the delay. I have conferred with Lynnette Taylor, the chair of the BANA Formats committee, who confirms my understanding of the intent here. The lapse here is an omission, not really a contradiction.
7.7.1c contains another layer to this exception of blank lines between interior and exterior box lines and Sample 7-3 is correct. When an interior (top) box line immediately follows an exterior (top) box line, there is no blank line inserted before that top interior box line. That is the exception stated in 7.2.1f. Omitted from this is the other half of this exception--when an exterior (bottom) box line immediately follows a (bottom) interior box line, there is also no blank line inserted. That is the general exception to blank line before and after box lines--when they immediately follow or precede EXTERIOR box lines, there is no blank line between.
Here is the second layer of that exception. When interior box lines (either top or bottom) are NOT followed or preceded by exterior box lines, there is a blank line as there would be if the interior box were not nested inside another box. That is what Sample 7-3 shows and this clarification should have been repeated elsewhere as needed.
I hope that helps. If it is still confusing, please let us know.