Perhaps they de-duplicate using smaller block sizes? Something like 256 bytes.
Also, if you believe Microsoft Outlook, there's something called "compressible encryption", which implies there are encryption schemes that aren't exactly random, meaning in turn that not all N-bit blocks are equally likely.
Also, if you believe Microsoft Outlook, there's something called "compressible encryption", which implies there are encryption schemes that aren't exactly random, meaning in turn that not all N-bit blocks are equally likely.