The sum of all partial pressures in the lungs must add up to the ambient pressure.
The composition of gas inside the lungs includes the vapor pressure of H2O at body temperature which is about 0.9 psi. This is independent of ambient pressure.
Inhaled air is also diluted by the dead space gasses from the previous breath, which is about 150ml out of every 500ml breath.
To compensate for these effects, space suits use ~4 psi O2.
https://medchrome.com/minor/anaesthesia-minor/lung-volumescapacities-and-dead-space/
https://vrchemistry.chem.ox.ac.uk/potential/Text/solutions3.htm