Terry,
I tried to respond to your reply last night via email but it didn't get into the thread.

The house was rebuilt in 2006 after Hurricane Katrina; the levee failure allowed 8' of water into the whole first floor.

The hinges are mortised into the door and the frame. The holes I drilled are now filled with 1" pieces of 3/8" oak dowel but before that, when I jammed a longer piece into the holes, it DID seem like there is a gap between the frame and the stud behind it. (I'm assuming the construction design you suggested; I know little about house or door construction.)

Based on Ed's comment, I'm going to leave the screws in and will now attach the hinges to the frame.