[Mp4-tech] H.264: difference between SNR value of encoder and decoder

[AngelZhang innosis.com.cn]

hi,Everyone
  I set parameter PicOrderCntType as mode 1, and found the output SNR value
of encoder is different from that of decoder with the 264 bitstream built
by encoder.
  and in my opinion, the decoder's working principle is same as that of
hypothetical reference decoder built in encoder. so i have no idea about
such a thing.
  please give me some interpretations.
thanks a lot!
best regars!
angel