Here is my understanding, let me know if I've got it right or wrong:
Until recently, most cryogenic propellants were at or close to their boiling points. Each unit of heat leaking into the tank would then boil off a corresponding unit of propellant and the temperature would stay constant. This is roughly like adding near boiling "make-up" water to a pot of boiling water. It might be a little cooler, but not much.
But in the case of sub-cooled LOX, the temperature is way below the boiling point, somewhere between 10 and 30 degrees C depending on the case. When heat leaks into the tank, it will raise the temperature of the LOX which then expands. A full tank would then overflow - remaining full but with less mass of LOX due to the density change.
There would be no boil-off to "make up" by topping it off. It would remain full but steadily decrease in density as it warmed up.
So for a rocket with first stage tank filled with well sub-cooled LOX, why would it need to be "topped-off" until the last minute or so before launch?
This video at
T -00:05:05, video time 06:52, my approximate transcription of the narration:
Right now we’re just coming up on the five minute mark here, we are concluding the loading of the RP-1 on to the first stage […] and we are topping-off liquid oxygen as well on that first stage, and we’re going to keep topping that off for about two more minutes.
The video is already queue'd up at the appropriate time code (in this case I think the video edit is final):