The Tom Jones response applies here ("It's Not Unusual"). Whereas the SAAMI specs call for 0.5550 inches at the web, in fact there is some tolerance built into the chamber. As the brass expands to fit the chamber, the web is the least tractable or ductile due to rigidity. Consequently, above the line that you see, the brass fits your chamber with the allowance for relaxation following firing. Below the line that you observe, the case dimension approximates the original dimension.
