Put another way, if there are 10,000 shrines to this one saint around Europe, the probability that 7 will randomly be in a straight line on the Mercator projection is a lot higher than if there are <10.
I don't think it's that low. The number of coincidences will rise exponentially with the number of churches. You also have to account for the fact that there are something like 1000 prominent Catholic saints, and 10's of usable map projections, etc.
