Randy like i said i was taking a shot at it! I was just guessing. Your explanation sounds dead on nuts to me!Not sure what you are suggesting. Precipitation of calcium carbonate on sand lowers calcium and alk.
Dissolution of enough sand to boost calcium to 660 ppm from 400 ppm would ADD 36 dKH of alk to the water.
So the sand seems an unlikely explanation.
Test error or overdosing seem likely.