MRI's are have a limit on the precision that they can measure size with. I don't know what that limit is, but the small difference you are seeing may very well just be within that lack of precision. Also, different Drs can come to different size measurements based on the same MRI. So, it is very possible that you have no growth at all. I would do what you are doing, get different opinions from your different Drs and radiologists and ultimately decide for yourself it has grown.
Even with precision measuring instruments there are varying measurements that can result. A micrometer is a very precise, to the thousandths of an inch, measuring device used in machine work. As a person with low experience using them, I can measure the same part several times and get several different measurements with them varying about 1/3 the thickness of a human hair. A skilled machinist would get a different measurement from mine. At some point one of those numbers, or a range like +/- 0.002 inches, has to be used.
I don't know if an MRI is precise enough to measure something like an AN to a mm here and there. I definitely would not be chewing fingernails down at this point as you may very well be told to wait 6 months or a year and get another MRI.