Last week during preflight checks of B747-300, I noted that Pax Oxygen pr was low. I sent tech to change the bottle(s) with low pr. He later on told me that three out of five bottles were having pr of 1100 psi each and rest of the two were having 2000 psi each. Going through previous tech logs, I noticed same three bottles getting replaced almost fortnightly. Question is if there is leakage in the system why all five bottles r not getting depleted , being in parallel ? Why these three ? Anyone out there to help me out ...