In Glow, your fertile window adjusts retroactively in response to your logs. These types of tracking are by nature "retroactive" in the sense that the temperature spike or the peak OPK result shows that you have ovulated the day before.
As you enter data into Glow, Glow uses it to calculate and adjust the fertile window to match what it has learned. The algorithm weighs all of the data you enter - periods, OPKs, BBTs- when making its predictions. Occasionally there is a conflict between the data, for example, an OPK may indicate you ovulated on one particular day, while your BBT indicates another, or you enter a period that makes it unlikely that the original prediction was correct. When that happens, the algorithm will calculate the most likely fertile window based on all the data together, and as a result, you may see the fertile window change.
This can definitely be very confusing, but it's all part of the process of the app working to incorporate your data to make the most accurate predictions possible. Glow needs to take your previous data into account to refine and improve your future predictions, so when these changes occur, you can think of it as Glow getting more and more well- informed about your unique cycle patterns!