In this answer I wrote:
In other words, a few tons here or there could depend on something as simple as the weather, or trying to squeeze a little extra propellant in for a given mission with a tight constraint on range, or where they want to land the reusable booster.
I think a difference of only 0.5% is too small to try to nail down to a fixed value.
Meaning that from one launch to the next a given liquid propellant rocket's fuel mass could easily vary on the order of a percent (or more).
While there are mass flow controllers for liquids which can account for density variation dynamically during a fill, LOX has a constant loss due to boil-off. Some boils off curing cool-down of the tank, and some is intentionally boiled off in order to refrigerate the remaining LOX. For more on that see @Uwe's excellent answer to Why would sub-cooled LOX tanks need to “topped-off” until the last minute or so?
Question: So how do they know how much LOX is in a rocket just before lift-off to say a fraction of a percent? Or do they in fact not know it to that precision?