From what I understand, prior to the launch, the relief valves are open to allow some of the boiling LOX to evaporate and be replaced with fresh, cold one (we can see oxygen evaporating - and "bursting", from time to time - from the rocket). When some (unknown to me) condition is met, the valves are closed and tanks are pressurized with helium. If someone could confirm or clarify that this is correct, it would be great, because I suspect I'm missing something here.

During the NROL-76 launch, we heard the "pressing for flight" announcement for Stage 2 at T-40s, and for Stage 1 at T-20s.

However, the time of closing these valves vary from launch to launch. For example, during the CRS-7 mission, S1 valves were closed at T-2:20 and S2 at T-50s. During Orbcomm-2 mission, both tanks were said to be pressing for flight at approximately T-1:00.

What is the reason these times vary from launch to launch? Is it because of the different LOX temperatures?