[awalton]

They might not even be able to measure that small of a difference in the spacecraft's acceleration, to be honest. We're talking about a difference in grams of a spacecraft that's somewhere north of 1000kg right now - wet mass was maybe 1529kg at launch [according to NASA; Lockheed said the bus could be up to 2000kg at launch, so I'm not sure who to believe being honest - it's a little hard to believe they'd risk things and short the mission on that much hydrazine], dry mass 860kg, split the difference and it's probably around 1200kg [or up to 1500kg] currently.

It's a little easier to believe they could detect whether or not they got 60g by the last significant digit change in the moment of inertia from their instruments by nulling out the rotation... It's a little harder to believe they could detect a difference between 0 and maybe 500g in the sample return container via their acceleration data. I doubt they even know how much hydrazine and helium is left with all that much precision - at more than hundred grams a second burned, it's easy to believe there's some amount of accounting slop.