Period predictions

Rebecca
Hi
​Glow has my average cycle length at 31 days, but for September is has it at only 25 days, so my predicted ovulation days are sooner than I thought they would be. How can it predict my cycle this month as being shorter? Does it go off my cervical mucous input data too? It says I'm out of my fertile window already but I'm only on CD13.
​Thanks 

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