KH and Cal, Salifert test kits

I've been using the Hagen test kit from the start and was sure that my Cal
levels were properly read. Then I ran out of KH solutions and not being
able to buy refills for this parameter, switched to Salifert.

Well, using Hagen I was getting low Cal levels and low KH. Used buffers to
bring KH and PH up.
but
When tested with Salifert, my KH is above 500 yet the KH is not even
registered, I mean, according to the instructions I have 0 KH.
Any suggestions on why my KH is so low when my Cal is above 500 and pH is
8.1?

Thanks!

iy