I think I found the issue. The old German "zoll" unit originated before the standardization of units between countries. So it could have originally been some arbitrary number "close" to an actual inch but not exactly correct. Over time it was corrected to the actual inch. At the time the ad on page 2 was created it should have been correct. See link for more info:
Zoll (Pre-metric Prussian) | Units of Measurement Wiki | Fandom powered by Wikia