Concerned as to why peak LH is varying month to month

I’ve only been testing using OPKs for a couple months so I haven’t established a set pattern as to when I get a peak just yet but this month is vastly different than last month was. I got a positive opk on CD 11 in comparison to last month when I got a positive opk on cd 14. According the glow I wasn’t supposed to ovulate until this Sunday so I was quite shocked when I saw those two dark lines on the opk.

I was like wow that’s one dark test line I guess I’m getting close to my peak—oh no today is peak! I’ve also been taking prenatal vitamins so I’m wondering if that’s been helping my periods and ovulation to be improved than it was before taking them. Just wondering if it was normal for it to be that big of a difference.