My guess is that it's not just any old "Runway 27", but it's runways where the coordinates of the endpoints of the runways have precisely the same latitude down to whatever precision is in the database. It's probably quite rare to have a runway that's this perfectly east-west.
Looking at the coordinates of the runways in the FAA database, they are all perfectly east/west within about 2 feet of deviation N/S. This would round to zero if the calculations were done as lat/long as single precision numbers.
Runway 27 just means a runway that runs east-west and you're approaching from the west. Landing the other way it would be Runway 9, it's a neat little system that gives pilots everywhere a surprising amount of information in a single name.
