why I can't get the same frame numbers?

 hi, I modify the example code from xvidcore 0.9.1, I use it to read
video images stored in file, then encode 1000 frames and write every
frame to encode file, then I read from encode file, decode frames and
write to decode file, I find it only decode 945 frames, the number of
decode frame is less than the number of encode frames, where is the
other frames. why!
any of you suggestion is appreciate! thanks!

walker