Perhaps it is the identifier of the pad used, and doubles as a kind of EOF signal?
1 in 11 million probability of the first and last blocks being the same by pure random chance. I'd guess they must be some sort of pre-agreed message sentinel, but even 1 in 11 million chances happen sometimes!
Right -- as improbable as that sounds, it pales in comparison to the likelihood of renovating your chimney and finding a pigeon with a coded message from WWII!