Hi and welcome to the Chiari forum.
What you have is Chiari Malformation I....with a herniation of an 8 or 3 or 4....which is really not as important as if it is creating a CSF obstruction...
It is possible to get different measurements...bcuz the Dr is not aware of how to do the measuring....or if they are looking at diff MRI's the angles and slices can be different....
It also sounds like you may have POTS,,,,,there is more testing you should have....have you had a recent MRI? to see if there have been any changes?
We do have a list of Drs for you to use to research Drs....the list is not a referral nor an endorsement....
http://www.medhelp.org/health_pages/Neurological%20Disorders/DOCTORS-LIST-FOR-CHIARI-SYRINGOMYELIA/show/1062?cid=186
Be advised you may not find a Dr in your area and may need to travel....many of us do...
Know you are not alone <3