For a simple experiment, use the "Flat" preset. Be sure to set the number of Trials = 10.
Now click the Resolve Combat several times. You will notice it varies from 30% to 70% fairly regularly. This means that even with a large numbers of combat units (80 total in this case), over a number of trials, there could be a wide variance in results. In other words, sometimes it will feel that you will be on long losing streak, and other times a long winning streak. Sometimes, you might think that the RNG is broken, or the AI is cheating. Nevertheless, the results are purely random.
The mind will find patterns in random events, but the patterns are meaningless and non-repeating. These phantom patterns reflect patterns in the mind, not in the random events. Indeed, if you could measure the (true) luck of everyone in the world, you would find that some people have been lucky, and some people have been unlucky. The good news is that one's past luck has absolutely nothing to do with one's future luck.