The drywall is shaved to allow the casing to sit flat on the wall and on the frame.
Door jamb not flush with drywall.
If you have or want to purchase a biscuit jointer they do a nice job of keeping your extensions sturdy and lined up with the jambs.
If not i think your best option is going to be to build the extensions proud of the wall and then bring them down flush with a power planer or belt sander.
2 basic time taken.
Nail through the shims and then trim the shims flush to the outside edge of the casing.
Flush grooved primed door panels are similar in that they.
However you can certainly use it to create a 2 tone look as well where the door panel is different from the wall color.
The casing door and window frame moldings should be mounted parallel to the drywall not canted in.
This is caused by over shimming the door jamb opposite the hinge side.
Flush primed door panels are ideal when you want your wall and door color to be the same.
The hinges or the edge of the door are making contact with the jamb prior to the door completely being closed.
While preparing a door frame for casing we notice the drywall is not flush with the door frame.
In other words the jamb is not at a perfect 90 degree angle to the door and the door hits the jamb not allowing it to completely close.