I work in inches but also use mm from time to time when designing for 3D printing.
Try this. Draw a line of say 3.123 in inches. Switch to mm to see length.
Go back to inches and change to 3.1231. It shows as 3.123 in inches but the number is increased in mm. MM to 3 places is much finer than inches to 3 places. So, internally the precision is quite high but the readout is rounded to only to 3 places.