Fill Line and Capacity for Kegs

I am hot packing syrup in 1/6 kegs with the Sankey/beer top, not the Cornelius/soda style. The official capacity stamped on the keg is 5.16 gal/19.5L, but I'm wondering if the capacity is measurably higher with the ball lock and tube removed and my filling just about to the bottom of the neck. There's a gap between what I've measured coming off the rig and the number of kegs I've filled.

Anyone have a more accurate capacity for their kegs. I imagine it's the same issue when using other keg sizes.