Here's a theory - the samples were pure aragonite and tested fine. The larger quanities also contained a whole pile of aragonite dust, which has disolved into your water, causing a temporary rise in pH. Do a big (90%) water change to flush it out, and see if the pH drops. If it does, you know it was a one-off and you don't need to worry. If it doesn't, test water from another tank to verify the accuracy (or otherwise) of your test kit.