[vollmond]

I would assume there's a floor on the size of waterway it starts with. I have the same situation (well, less so) in that my backyard is 3 blocks from a named creek on the map, but it started me several miles away on the larger creek that my local one flows into.

Edit: also seems to generalize your location - pretty sure it's just figuring out the town I clicked in, then going from there.