Kayak is not a booking site, it's just a referral site (like Google Flights) that redirects you to airline/OTA sites for booking which are most likely just published fares. Just did some searches on Cheapoair and Justfly (DTW-FCO and DTW-TXL for example), and it just returned the exact same published fares that I can find with Google Flights. Guess I'm missing it. Perhaps you could give an example. Not really convinced what you are seeing are necessarily bulk/consolidator fares. Often you can save money by booking with a different marketing carrier coding on the flight even though the flight itself is the same. These are still published fares. delta.com will only return DL marketed flights when using it's fare search. You often pull up the cheaper fares on delta.com by using ITA Matrix to search and then ITA Powertools or BookWithMatrix to pull them up on delta.com