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

Why this happens?can you guys explain me?