How does microscope calculate the micron/pixel ratio? any specific formula behind this conversion?

How does microscope calculate the micron/pixel ratio? any specific formula behind this conversion? Which factor changes the micron/pixel ratio. Because i acquired images at certain magnification and micron/pixel ratio shows the same for each images but when i make mosaic of all these image it shows different micron/pixel ratio? why ?

Example

Micron/pixel ratio for the every each images at 20x magnification→5.78

However when i make mosaic of these each images, micron/pixel ratio changes and shows the → 10.12

But when i acquired other objects at same magnification then tile image micron/pixel ratio slightly changes like 10.13,10.12,10.14 for every object. While each images shows the same constant um/pixel ratio as previous 5.78. If magnification value is the same then it tile image should show the um/pixel ratio 10.12 for the other object? why this value slightly different for other object?
Why this happens? can you guys explain me?

“The microscope” is a little bit vague here. It sounds like you are using something where the magnification can change, i.e. if you zoom.

In general, the pixel size for microscopes/objectives is fixed based on a manual measurement (slide of some sort with objects of known size).

It may be that you are choosing to downsample when creating your “mosaics”, though I am not 100% sure what you mean by that term. If you are referring to tiling, then you might have chosen to downsample to reduce file sizes. I am not sure.