the output of ls /var/lib/apt/lists/*.debian.org_* | wc -l is not constant

[Regid Ichira <regid23@yahoo.com>]

  The output of that command varies by a +/-1 in the last couple of days.
Why it that?  Could it be related to security updates in stable?
  The listing itself is related to the contents of /etc/apt/sources.list.  My 
sources.list uses:

    deb security stable/updates main contrib non-free
    deb-src security stable/updates main contrib non-free

    deb unstable main contrib non-free
    deb-src unstable main contrib non-free
    deb testing main contrib non-free
    deb-src testing main contrib non-free