I have chiari malformation. some of my scans read 5 mm. two read 3 mm. one reads 2mm on one side and 4 mm on the other side. what causes such different readings?

Different machines used to scan? Time of day? different times in eating and scan? Differrent angles when you were on table. Difference technique? Discuss with the team that did them.

You do not describe why you have undergone multiple scans for what appears to be a mild Chiari malformation. In the absence of actual films, it is not feasible to over-read. The doctors who are involved in your care should be available to discuss clinical correlations and further steps.

A Chiari malformation is defined as at least 5 mm of tonsilar herniation. Some of the differences could be scan technique.