Well, that is certainly strange.

Instead of playing the "20 Guesses" game and going back & forth on speculation, I think the appropriate thing would be for you to post a complete set of water chemistry data for both your pool water and your fill water. Having been on this site, you know that we all prefer test results from a Taylor K-2006 test kit. If you can post the full suite of test results and update your signature with details about your pool (size, equipments, etc), then I think I and other folks here can help you better. Short of that, all it will be is conjecture...