The original airing of the Susan's birth episode ran 90 minutes on ABC, but in syndication, they have to have all episodes running one hour apiece. Problem: when you split up the 90-minute episode into syndication, you get a reverse time problem; instead of having to edit out 3-4 minutes for more commercial air time (gasp), they needed to find some footage to pad the two parts a bit in order to bring it up to about 45 minutes total time without the ads per hour.
Hope that explains things better.