chemistry aqa aslevel %uncertainty question

Each reading on the balance used to record the mass of the fine needle syringe and contents had an uncertainty of ±0.001 g
Calculate the percentage uncertainty in the mass of liquid A injected in this
experiment. https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2020/november/AQA-74041-QP-NOV20.PDF page 20
answer = ((2 x 0.001) / 0.460} x 100 = 0.435%
why did they use the difference in mass (0.46) instead of adding up both masses?

Because they wanted to calculate the mass of liquid injected from syringe
So they measured the mass of syringe with liquid - 11.295g.
Then measured mass of syringe with some liquid remaining, after injecting - 10.835g.
So the mass of liquid injected = (mass of syringe + liquid before injection) - (mass of syringe + liquid after injection).