Apps utilize your own historical cycle data to estimate your fertile window. Using retrospective information, they anticipate your cycle will follow the same pattern as previous months. If you are new to using tracking apps, they use predictive algorithms based on population data, not data personal to you.
Unfortunately, our bodies are not that predictable, and they do not always fall into the norm cycle pattern that population data assumes.
Your follicular phase can vary each month, which means you do not predictably ovulate at the same time each month. The best way to accurately catch your true fertile window is using a combination of tools to analyze your personal body signs. Analyzing your cervical mucus, utilizing LH strips, and taking your temperature first thing in the morning with a BBT thermometer will help you hone in on your true fertile window.