calcium reading

Well Yes the more synthetic salt you add the higher all elements will be. We commonly call the mix "Salt" but it is far from just salt. It contains a ton of buffers to closely match what the manufacture believe is Natural Sea Water.

 

Agreed, a hydrometer is useless and caused no end of problems in my tank a couple of years ago.

If a hydrometer is used and is completely out, salinity can be high (but read normal using a incorrect hydrometer) which also means that calcium will be high, magnesium in salt water is found at a higher concentration; it will be higher but not noticeably higher.
This can cause a lot of problems which I found out years ago; first hand. Namely corals dying for no reason, fish jumping out of the tank and easily contracting disease.
If you use a refractometer to measure salinity and a good salt mix with RO water there is no reason your calcium levels would be high without adding some sort of supplement.