What is "extra timer interrupt" in powertop output?

[Andrey Borzenkov]

After disabling CD-ROM polling the next top-100 winner is "extra timer 
interrupt":

Cn                Avg residency       P-states (frequencies)
C0 (cpu running)        ( 4.2%)          750 Mhz     0.0%
polling           0.0ms ( 0.0%)          500 Mhz   100.0%
C1                0.0ms ( 0.0%)
C2                0.9ms (95.8%)


Wakeups-from-idle per second : 1101.2   interval: 5.0s
Power usage (ACPI estimate): 11.7W (2.2 hours)

Top causes for wakeups:
  31.6% (  8.4)       <interrupt> : extra timer interrupt
  15.0% (  4.0)   hald-addon-acpi : schedule_timeout (process_timeout)

I assume, this is what is simply called "timer":
{pts/1}:~%> cat /proc/interrupts
           CPU0
  0:    9060975    XT-PIC-XT        timer
  1:      18742    XT-PIC-XT        i8042
  2:          0    XT-PIC-XT        cascade
  3:          2    XT-PIC-XT
  4:          2    XT-PIC-XT
  5:          3    XT-PIC-XT
  7:          5    XT-PIC-XT        irda0
  8:         11    XT-PIC-XT        rtc0
  9:       3376    XT-PIC-XT        acpi
 10:          2    XT-PIC-XT
 11:     239251    XT-PIC-XT        ohci_hcd:usb1, yenta, yenta, yenta, ALI 
5451, pcmcia0.0
 12:     621206    XT-PIC-XT        i8042
 14:     225993    XT-PIC-XT        ide0
 15:     315463    XT-PIC-XT        ide1
NMI:          0   Non-maskable interrupts
ERR:          0

Any way to find out why there are so many of them? System is using dynamic 
ticks:
{pts/1}:~/src/linux-git%> grep NO_HZ /boot/config
CONFIG_NO_HZ=y

TIA

-andrey

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