Stamp Smarter just published a new article on US Line perforations gauge measurements by Julian J. Goldberg.
Go here and then click on link next to "New" icon
http://www.stampsmarter.com/1847_landing.html 
In the article look at the image for the Line perforator machine. At what accuracy do you think that those pins are aligned to each other? And over time, imagine how the pins might be bent or otherwise not retain their relative positions to each other.
Don