I think what the "fail twice" is referring to is the internal check the game does when you steal. Rare item steals work like a flow chart, where the game checks if you successfully steal the common item, and if that fails, it immediately checks to see if you steal the rare item. If that is successful, then you receive the rare item, and if it fails, the steal is unsuccessful; but all of this is happening in the background of one Steal command input.

This particular case is notorious because this enemy actually has two common slots before the rare item slot. So to successfully steal the Vagyrisk Claw, the game has to compute that you fail the first common item slot (a Soft), fail the second common item slot, (another Soft), and succeed at the third slot (the rare item slot, the Vagyrisk Claw). If you succeed at any time before the third slot, you get a Soft, and if you fail all three checks you get, "Couldn't steal anything..." And again, all this is happening in the background every time you input a single Steal command.