"Bullet holes" on stamps

[JohnBenn 10]

"Bullet holes" or the white ring you see next to the "1" in the value tablet.

Does someone know how these are formed?

The global consensus is that they are formed by ink bubbles bursting,but when they appear on the same areas and at the same size it doesn't seem such a very plausible explanation to me.(The also occur on stamps of different countries)

 

Does someone have another explanation?

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Edited October 5, 2012 by JohnBenn