Repeatability and observability. Yes, you can come up with a theory and go check and see if what you predicted is there. But even if it is there, that doesn’t prove your theory. It could simply be a coincidence, caused by other factors you didn’t consider. The opposite is true. Your theory could be correct, but outside factors you didn’t consider could cause you to fail to find the evidence you expected. The past is not repeatable, and that is the fundamental distinction between operational and historical science.
Yes, unknown mitigating factors can influence your measurements. That is exactly why repeatability is so important. By having others attempt to get your same results, you lower the chances that your results were a fluke.
Popper’s idea of falsification was nuanced. He understood that flukes and errors can happen. This is again why repeatability matters. Keep doing it. Have others do it. If that anomaly you reported keeps happening, then yeah, I would consider the possibility it has been falsified, unless there is some factor you can isolate that was causing the anomaly.