I might be able to explain it, but better just to give you a link to the authoritative source:
Lid Camera Calibration Beta
There’s still a problem when selecting artwork that it increases in size relative to the image of the work piece but after de-selcting any art it seems to be in about the right position. (Insert obligatory rant about unavailability of Snapmarks here.)
Does the story also hold that immediately after doing this he mumbled something about “I lose more bowstrings that way”?