Why are months (e.g. December) used to notate a runway?
Very likely, whatever data source you're using has been mangled by Excel (or some other spreadsheet or data processing system that tries to convert everything that might be a date into a date format).

26-Aug is actually runway 26/08, 30-Dec is actually runway 30/12, etc. (See "How are runways numbered?")

Some computer system saw those numbers, assumed that they were dates, and presented them that way. The other runways aren't valid as dates, so it left them alone. It wouldn't be the first time that people used Excel without fully understanding its default date-parsing settings and thereby causing problems for people trying to use their data afterward.