Did you actually book the United flight through Kayak, or just shop around?
A few years back, did a search on Orbitz (back when I always used Orbitz for searching, and sometimes booking). Found a flight I liked. Clicked on the "buy" button, and went through the pages. On about the 3rd or 4th page, when it finally was at the "click here to buy - no turning back" page, I clicked on it....only to tell me that the flight was no longer available. (and no, it wasn't one of those "just 1 ticket left at this price" flights...just listed as an ordinary flight)
I had also checked the mother ship's website, and had been surprised to not see the flight on their website that Orbitz had listed - so when I was denied on Orbitz, I wasn't too surprised. I then learned the hard way that not everything you see on Orbitz is necessarily available.