How to determine the length of the RBF file

I used the decoding scheme given in the above quoted bootloader source to determine rbf respectively configuration bitstream length and found, that the length doesn't match exactly. Unlike the exact length information in Cyclone II rbf-header, I got a length with a few bits remainder and by zero to three bytes shorter than the actual rbf file. I wonder if the decoding scheme may be partially incorrect. Because it doesn't hurt to send a few extra bits, I adjusted the length empirically. Does anyone know more about this issue?