It's the free weekend. After the weekend is over (Monday at 10am US PST), try syncing your account again to see if it falls off. If it doesn't, it's Steam's fault because it's still listed in your recent games list. It might take a while, but that's why the giveaway is open for 2 weeks. I had the same thing happen to me after the Payday free weekend. All you can do is keep trying. :)

Same thing happened to me with Serious Sam HD The Second Encounter, only had the demo and it thinks that I have the full game. Now this game, tried it out to make sure it works alright on my laptop and here it is again, "Exists in Your Account". BTW the Serious Sam one is still in my account sync after 5 months.