As I posted in a thread last fall:
EFO = an acronym for error, freak, oddity.
I visualize this on a scale of 1-10, let's say:
10 - error
5 - freak
1 - oddity
Each collector gives a mental score to any EFO item with price/value increasing accordingly up the scale. There are no set definitions, as you can see from the responses you have received so far. Some have called your stamps an error and some have not.
Owners tend to rate their items higher on the scale than bystanders would. I would rate a minor perforation shift such as yours at about a 2. Others may disagree, and probably will here shortly!