Not sure why exactly. But I've done it on both my 4S twice and my iPad 2 thrice to see if it was just me messing up the steps. But in all 5 trials, the result was the same. I had to repeat it a second time in order for it to work. The error message that comes up is that the restore has failed because you probably guessed the FW even though it's correct, the 4S is using the iPhone4,1_5.1.1_9B206_Restore.ipsw, and the iPad is using both the iPad2,4_5.1.1_9B206_Restore and the iPad2,4_4.3.5_8L1_Restore.ipsw, both are correct for the respective devices.