The pressure relief valve does set the pressure - but only against a 'dead head' presented by the rest of the engine. In other words, the pump will stop making pressure at 70PSI only after it sees a 71 PSI burden ahead of it. And in creating that 71PSI burden, more oil must flow as a result.
I agree aeration might well be another factor...that's why I said I didn't want to parse all the details, only the end result which is a loss of pressure due to no oil to suck. However it happens, I think it happens. Not in all engines, but some.
