If anything, it's a LACK of ink in that spot with the paper color showing through. (The numbers on these are actually created through the negative space of the blue ink.) Could have been a bubble in the ink that caused that. There are TONS of examples of this happening on the fronts (I think the glossy side was a lot more susceptible to ink bubbling than the backs, but did still occur).
Also, if you click on the image you posted (it enlarges it), you can see a slight difference in the way the paper looks in the normal number area and in that circle of missing ink.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).