Inconsistent tumour measurement

Hello - I have recently had an ultrasound to determine if my tumour is responding to chemo. I am due to have my 3rd tax next week.The ultrasound measurement is significantly smaller than the oncologist’s measurements. He measured using a tape measure and calipers. The lump is big and to me it’s size seems more in line with the oncologist’s measurement but surely the ultrasound should be more accurate?

The scanographer told me it has had a good response to chemo, but I don’t want to get my hopes up when I can feel that the tumour is bigger than the ultrasound measurement. Does anyone know why this might be? Could part of the lump I can feel be partly made up of something other than the tumour?

Am really hoping the scan is correct, but scared of getting my hopes up just to get them knocked down again.