Those are the stats provided by [words=https://officialhydromaxpump.com/?uid=6&oid=2&affid=98 ]Bathmate[/words] in the users guide: 0.4 bar in water, 0.17 bar in air. Air is compressible and expansible, water is not, it doesn't seem to make sense that it would effect vacuum and pressure characteristics the way it does, but in actual practice that's how it works.
I remember testing relief valves with a dry nitrogen set and they all worked IAW spec; then an old pro told me to put 'em on the flow bench using the same oil they were designed for= all outta spec. And that's 'cuz two different fluids, liquids and air, have different volume and pressure properties.
Kinda counterintuitive, but that's how they are.