[mrtg] Why "Max In" is less than "Current In" in "Daily' Graph?

[zhuergang 03325]

Hi, everyone! 
  I use MRTG to draw pictures of router's traffic. But unexpected thing happened.
  The "Max In" is less than "Current In" in "Daily' Graph (5 Minute Average)
". It's unbelievable. It looks like this:
  Max  In: 272.0 kb/s (0.3%)   Average  In: 260.0 kb/s (0.3%)   Current  In: 277.6 kb/s (0.3%) 
  I think maybe it's MRTG's bug and I want to know why and how to resolve this problem.
  Thank you very much!
                                      Eric Zhu