Here in New England we have loads of odd-distance courses. Most of them are either landmark to landmark or, more frequently, courses that were measured by car (or maybe horsdrawn carriage) odometer to what was thought to be a standard distance, then later found to be off when certified, but by that point had so much history the organizers did not want to lose that (the 47-year-old Westport, CT Summer Series falls into that category).
I've measured those courses and in fact one of the Westport races comes out to just under 3.1 miles, but there's no way I would ever allow it to be billed as a 5k (although many local runners claim their "5k" PRs on it).