score:3
From Kayak's help page on prices:
Although we try hard to find accurate prices, sometimes we don't. Fill out the form below, and it will alert our programmers so they can investigate the problem. With your help, search results will get better for everyone.
Kayak is not a perfectly 'live' search site. It's as close to live as they can manage, but prices do change all the time, and they could either hit all the airlines' sites every 5 minutes for every possible flight, seat, class and leg and combination, or they can provide ranges, estimates and updates 'regularly', if not via an API.
Also, some airlines don't include all their charges (credit card etc) in Kayak. RyanAir flights, for example, are notorious for this.
Long story short, Kayak is a pretty reliable guide, and some flights these days are even bookable without leaving kayak's website, but they're not perfect.