nearly same happened to me. i only have 2 computers one is macbook air other is sony vaio. and it says i have autrhorized 4 computers and i never plugged my iphone to any other device other than my two laptops....dunno what to do?...
In order to see which devices are authorized on your account:
Click View Account
Enter your password
Look in the second section iTunes in the Cloud
To the far right you'll see Manage Devices
Once it's clicked you will see all of the devices currently authorized in iTunes/on your account
I was going to say, "Hope this helps," but based on the above, I'm pretty sure it did!