I set sigma a to 0.1mm and sigma b to 1mm (this is the accuracy of my instrument) and got the following results. Then I set sigma a to 1 and sigma b to 1 and got the following results. What is correct if it is assumed that sigma a priori is 1 and the instrument's accuracy is 1?

When I set sigma a to 0.1mm and sigma b to 1mm, the sigma h values for each measurement are between 0.2 and 0.4, while when I set sigma a to 1mm and sigma b also to 1mm, I get sigma h values between 0.8 and 0.9.
Whats is correct?
Thanks.