Go to Ebay and run through fifty or so better US commemoratives that are offered for sale from the 1893-1930 period. This was the era in which the United States produced our stamps through a printing process known as flat press printing. In flat press printing most of these issues were printed in large sheets of 200 stamps which were later perforated and cut into sheets of fifty for shipment to Post Offices for sale. The details for each issue varies slightly, but for the US Columbian Exposition issue there were no perforations along the outer edges of the sheet of two hundred. This left each pane of 50 (that is a quarter sheet of the 200 pane that had been printed and then separated) with ten stamps in one row with a straight edge and five stamps in the the other row straight edge with one stamp being the corner stamp and having a straight edge on two sides.
This meant that on the Columbian Exposition issue that fourteen out of every fifty stamps (or 28%) were straight edge. So go to Ebay-bet you can't find 5% straight edges or stamps described as being reperforated. Collectors in the earlier period didn't like straight edges so unscrupulous dealers obliged them by reperfing the offending edges to make them appear perforated on all four sides. Some time reperfing is easy to see (the $5 Columbian above is reperforated on the left hand side). But most of the time it is not so easy. But most of all, collectors have, for the most part, stopped caring. Very quality conscious collectors still care and certifying boards still point out reperfing on certificates (when they can tell), but mainstream Ebay buyers don't seem to care much because they grew up in an all sides perforated environment and don't think to question whether the perforations are real or not. This same phenomonon is happening (and will continue to happen) with gum. Collectors only want NH stamps, so that is what is being supplied. Eventually regumming will be so ubiquitous that only a small set of highly motivated, condition conscious collectors will know the difference or even care. This is not good, but is part of the satisfaction with mediocrity that seems to happen with so many things.