Glow prediction accuracy

Carey
Glow is predicting that I'll start my period on the 15th, which would mean a 28 day cycle, but my average cycle length is 25-27 days.  I just scheduled an HSG test for a day based on glow's prediction, but I'm feeling signs of AF showing sooner than Friday (ugh, hopefully not).  Has glow been accurate for you?  Why would it suddenly predict a longer cycle for me?