I’m not sure about Kayak.com but I’ve found out that Kayak.it isn’t just an aggregator of flight fares, it’s even better… it gives you special discounted prices! When I was looking for my flight from Florence to Amsterdam I went directly to the airline sites, in addition to Expedia and Volagratis. They all had more or less the same rates. But when I performed the same search on Kayak it gave me a much better rate on the volagratis.it site. I then found out that accessing the vologratis.it site through Kayak you automatically get a 25 Euro discount.
Just so you know