/proc/stat cpu idle task time is reduced in VE.

I am using 2.6.32-042stab044.11.
I found vmstat threw floating a floating point exception in VE and the reason.

/proc/stat cpu idle task time is reduced in sometimes.

I can reproduce following steps:
 1. vzctl enter <ctid>
 2. execute following python script
root@ubuntu-64:/# cat proc_stat_problem.py
import time
while True:
   f = open('/proc/stat')
   for x in f:
      if x.startswith('cpu '):
         print x.strip()
   f.close()
   time.sleep(1)

root@ubuntu-64:/# python proc_stat_problem.py
cpu 13367 0 5512 494801 18113 0 0 0
cpu 13367 0 5512 508076 18113 0 0 0
cpu 13367 0 5512 508115 18113 0 0 0
cpu 13367 0 5512 508154 18113 0 0 0
cpu 13367 0 5512 495215 18113 0 0 0 <= D'oh!
cpu 13367 0 5512 502007 18113 0 0 0
^CTraceback (most recent call last):
  File "proc_stat_problem.py", line 8, in <module>
    time.sleep(1)
KeyboardInterrupt
root@ubuntu-64:/#
 3. check above 5th field of the result. cpu idle time should be increased but it seems decrease.

BTW, I cannot reproduce following command line.
root@ubuntu-64:/# while :; do cat /proc/stat | egrep '^cpu '; sleep 1; done