This happens because when you test, you get a snapshot of your LH. For example, on day 1 of your positive OPK, you might have caught the surge on it's way up. On day 2 you caught the surge. On day 3 you caught it on it's way down. I almost always get 3 days of positive OPK's and usually OV on the 2nd or 3rd day I have the positive. This sounds confusing, but this is what happens. On my second positive OPK, I've caught the surge. 12 hours later, I ovulate. The following day I catch the surge on its way down but I actually already ovulated.
There is really nothing to be concerned about unless you are getting positives for many many days in a row. This site has lots of good info about OPK's https://www.peeonastick.com/opkfaq.html