I'm trying to figure out how Glow decides when to start counting days past ovulation. It seems a bit confusing.

I took an ovulation test on CD11, it was negative. CD12, 13, 14 and 15 were positive. I didn't test on CD16. Glow marks CD15 as DPO0. Why is that? Isn't it more likely that my ovulation occurred on CD13, meaning that day 0 would be that same day, not 3 days later?

Any ideas why Glow is so off on DPO counting?

I'm now at CD21 and trying to figure out when I can test. Per Glow, I'm at DPO6, but I don't think it's accurate and I think I'm at DPO8. My cycle is anywhere from 24 to 28 days.