I believe I may have found a "clue". I just noticed a second #4 shell holder on my bench (#4 is used for 380 and .223, which I also load for). The second shell holder will allow the die to come down on the case about 0.002 - 0.003" further based on my measurements.
I'm betting I've gotten those shell holders mixed up and have been using the wrong one to size my .380 brass.
I'm going to run some brass through the newly found one and see if it sizes the cases differently. I'll bet it does.
