Closer to the airport, the IFR aircraft requires a step-down to get within sight of the field. In this Transition Zone, the base of Class E Airspace is lowered to 700 feet AGL.
Class D airspace generally does not have a shelf, and usually 700 feet vs 1200 is due to IFR approach zones
Yes, in the past. But thats why I said look at this online map and dig deeper. Its not just approach zones. The dotted boundaries inside some Class Ds are very gerrymandered based on neighborhoods and business areas. They are carving this up for drone regs and getting it ready for drone flight waivers to be issued automatically via the website instead of by hand currently.