I feel it must have something to do with cellular activity. My town has one very popular park with 2 gyms. The town next to me has another bigger park with a similar popularity but they are more spread out because of the size. I haven’t done any large scale tests but I am in the towns a similar amount of time because they are about 0.2 mile walk down this path from each other. I can see a gym from both from this path. But I seem to see more raids, more legendary raids in particular at the popular park. It still isn’t many at all, in a week there are usually around 5 legendary raids. What patterns have you seen?
This may help. Not sure if the algorithm for raid egg distribution has changed since this was posted though;