If you like conspiracy theories, another explanation is that by changing the flight number frequently for a given city pair, COdbaUA does not need to report ontime statistics for that flight to the Department of Transportation, as the flight has "insufficient history." I'm not sure I buy the "more flights than flight numbers" theory, as I'm guessing COdbaUA today has fewer total flights than PMUA 10-15 years ago, and PMUA kept fixed flight numbers for literally decades on some city pairs -- who doesn't remember UA 155/157 near dinner time for ORD-SEA, etc. etc.