the JavaFastPFOR version that we used compresses an array of N integers storing the value N into the compressed output OUT. At decompression time, the library takes OUT as a parameter and decompresses it using the N value stored into OUT.
We compress a posting list in blocks, which have a fixed size (e.g., 1024 postings). Also, we know the posting list length. So, storing N into OUT was unnecessary.
After ECIR'14, Lemire added a new interface to his library, which exactly addresses this aspect:
(as said be Matteo Catena)