The floor plan for this bedroom door has an unusual dimension-line from the face-of-stud at the left-hand side to face-of-stud on the inside of the bathroom wall at the bathtub…of 3’-8-1/2” or 44-1/2”.
In this particular example, the bedroom door was 2’-8” wide and the door casing was 3-1/2” wide. This requires a minimum width from drywall-to-drywall of 39-3/4 inches as shown in fig.
If the bathtub wall is 2×4 framing (3-1/2”)…then doing the math using ¾” thick drywall because of taping mud at the vertical inside wall corners…we have only 39-1/2 inches width when we need 39-3/4 inches…44-1/2” minus 3-1/2” minus 1-1/2” equals 39-1/2”. This would then require ripping 1/4-inch off one of the vertical door casing sides.
In this case, the width from drywall-to-drywall measured 37-1/2”. Either the bathtub wall was framed 2×6 or the framing layout ended up on the wrong side of the chalked layout line. The two drawings below illustrate both sides of the door casing cut to narrower widths in order to fit.