You are resizing some brass that was fired in a larger chamber, and the brass swelled at the base. The die displaces brass down to the limit of it's resizing capability. This leaves a the "ring" at base.
Risky business picking up .40 brass off the floor, and not knowing what it was fired in, or how many times. Especially depending on what pistol you are reloading for, and at what pressure?
You may want to consider sorting your brass, and pitching any that look like excessive swelling on one side of the base.