Worked it out!
We’ve got scales that can do volume as well as weight, so was weighing using them, and it’s only just dawned that 123ml of 5wt oil will weigh more than 123ml of water/milk/other cooking ingredient. I used the scales again to measure out my oil, then put the oil into a measuring jug, to find out I was actually trying to get around 160ml of oil into my fork!
Thanks for the replies, everyone.