* Inaccurate thermometer (are all times longer than expected?).
* Something in tap-water damages XTOL. (BTW, I use distilled water because my tap-water is hard).
If you don't mind running another test, you could do so using your times and process that are known to work. But AndreasT and you have already reported upswept curves, so maybe we already know what the result of another test would be...