Expressing: different bottles, different measurments!!

[Olivetti]

Has anyone noticed discrepancies in measurements when transferring from one bottle to another, even with the same brand? I express into Avent bottles, but sometimes transfer from a small to a bigger bottle and seem to see completely different measurements. Also, I just expressed into an Avent bottle, transferred into a Medena bag for freezing, and it measures WAY more according to the bag!

I have a medela pump and there is a noticeable difference between the amount I pump and the amount that shows if I transfer it to a medela bag for freezing so I don't pump straight into the bag any more but into the original bottle so I trust the amount it says.

The bags need to have the bottom fully opened out to be at all useful at measuring fwiw. But yes even bottles seem slightly different!