jagr - heaven only knows what they mean! I complained for years about the different methods of measuring runway friction and that often you did not know which was being used. I think there are at least 5 different Mu measuring devices.
That report does not make a lot of sense to me and I would probably 'go' on the 'medium' bit of the report if I were inbound. I would be tempted to think they were using CFRI units, but less than 55 would be 'fair' and 'medium' does not exist on that scale. As Denti says, in the EU a reading of 64 is WELL above 'good' (as is 53) using mu-meter readings and 'Medium' does exist there! Confusing, or what?
I think the best thing you can do is to ask your Ops Department to find out from EDDM - I would be interested to hear the answer!