The third picture from the top is by Mike Farrell, not Eric Oszustowicz, and I take exception to your use of the term "Photo courtesy of Mike Farrell," as I never gave you permission to use my pictures.
As for your question of "what's the difference," you have shown examples of two series of cars, each in their as-built and current conditions. The first picture shows an as-built 2200-series car, the second shows a 2200 as rebuilt (the tilt-sash windows were added in the rebuilding), the third and fourth show as-built 2600-series cars, and the fifth shows a 2600-series car as rebuilt.