Sorry Barrie - it was right there in front of me. The differential? Well, one is 3 times bigger than the other, or 200% increase / decrease depending how you look at it. But at such tiny amounts science would say that the difference is so small as to be insignificant, which is why most medics (and NICE) would view anything less than 0.1 as undetectable.
If the lab has two machines, these will be calibrated every so often but there might be a possibility in between times of one machine reading higher than the other. Calibration is arbitrary anyway, doctors in my area know that sending the blood sample to Harrogate results in a higher PSA result than if the same sample is tested in Leeds simply because the machines are set differently.
John's specialist says that if you put the same sample through a machine twice there can be as much as half of 1 tenth difference in the result - this is the tolerance value - so the same sample could come back at anywhere from 0.01 - 0.05 or 0.03 - 0.08. For men with higher PSA, this matters not a jot as their result is often given in the nearest whole number anyway so no-one knows whether it is 21.5 or 21.9 but for men having ultra-sensitive tests in can cause unnecessary worry.
I am sure you aren't really rubbish at maths but if it is true, are you absolutely certain about the readings you are getting .... you haven't shoved in an extra zero by mistake? 0.1 and 0.3 are quite another matter!!!! Whatever, I hope the MDT discussion leads to some peace of mind for you.