Can someone tell me why?

lizzie • Wife 👰🏻 30. TTC 👶🏻 #1 since mid October 2018. Wales 🏴󠁧󠁢󠁷󠁬󠁳󠁿

Can someone tell me why glow has moved my ovulation day from today to yesterday? I got a positive opk around mid day yesterday, and glow said today would be my ovulation day which all makes sense! Then today I have taken another opk and its fading and my ovulation date has now moved back to yesterday!