Usually they allow for some variability, so the typical limit starts at 8 mm. And I say starts because they add an additional 1 mm for every 10 years after the age of 50 years. Also, if the patient has had a gallbladder removed, or something that would compromise the reserve capacity, then the upper limit can go up to 11 mm or so.
Another thing to consider is the variability of the measurements involved with the imaging. In your case, the radiologists may have looked at your bile buct from a slightly different angle, and maybe measured in a different location. But there is the very real possibility that they did measure a real change, and that there was something going on to cause the 11 mm measurement of the diliation.
If you did have an obstruction that caused the diliation, it will also tend to cause a bile flow backup into the liver. The pooling of this bile around your hepatocytes will lead to damage and elevations of your liver enzymes.
Only your doctors will be able to diagnose you of course. Hopefully it will be something simple, like a passed gallstone, that doesn't need any followup treatment. Take care.
Really good answer!! Just wanted to comment, thanks for this!