I spent a LOT of time last night and tonight trying to get as close to the canonical "optimum" dV transfer from Jool back to Kerbin. This was the projected trajectory AFTER I had completed the Jool departure burn. Note the periapsis very close to Kerbin's orbital plane.
However, immediately after leaving Jool's SOI, having made no maneuvers or any kind of trajectory adjustment, my vessel was tremendously far off the trajectory projected after the departure burn:
The orbit a