the terminology is different - in stamps this is considered a freak
Not in this case. As John Becker said above, the perfing machine was set to allow extra margins all around the outside of the printed area on both panes. As a sheet came into position for perfing, it was natural that some of the marginal printed material would be "captured" along with the stamp design. The nature of the perfing process allowed for the sheet to "float" within the range of the perfing wheels, so the amount of capture varied from one sheet to another. It is nice to observe this, but it was not due to any malfunction. Captures of various kinds are a kind of sub-specialty for some Banknote specialists.
Quite a few years ago Jack Reinhard wrote an article on this subject and had it published in the old journal Strictly US