If you can find someone with a Ca meter I would ask them to test it.

I had a similar readings with Ph. I was reading 8.6-8.7 with my two hagen test kits and 8.1 with the salifert. Irritated I bought a Ph monitor and my water is 8.2. However, on another tank all three test kits read the same as the Ph monitor. I don't understand that but something has to be in the water messing with the hagen test kits.