That was my first thought, temperature change of the oil changes the volume. When im evaluating an inground to determine if its leaking underground I take the temperature of the jack into account. Ideally leave it off overnight and let the oil cool and set everything up next morning then come back the following morning to check.
Also if your trying to determine the amount of oil lost from the packing get a marked measuring bucket from the paint area at your home goods store to use as a pit can. There's also quick volume calculators online to convert that volume into what you should see in vertical movement of the piston.
