This is about the threat range for a critical hit. One of my player has a Keen Rapier (18-20 threat) and adjusted with the Keen ability it is 16-20 threat range. I vaguely remember reading somewhere that the threat range was the difference between the two numbers (ex: 20-18=2) so this would be the difference with the Keen ability of 4 or 6 numbers in the range.
Thanks
Thanks