Why I always see the content changed --- use similar(0.99) instead of exact()

I am trying to monitor a website to see if the content is changed
But I always got "content changed" even nothing changed from my eye (all content is the same include picture, text..etc)
(It seems the website will keep updating every sec even content is the same)

Below is my sample code

r= capture(Region(776,142,65,17))
sleep(5)
if exists(Pattern(r).exact()):
 print "content is the same"
else:
 print "content changed"

I always got "content changed" even nothing changed.
I try to remove Pattern but result is the same.
I think the problem may be caused by website updating every sec.

I also use "wait()" and "waitVanish()" instead exists(). The result is the same.
Does anyone know the problem or any suggestion for me?