Sheet-fed offset printing quality can’t be beat, but as with any mechanical process, printing, trimming, and binding all exhibit miniscule variances (usually under 1mm) from piece to piece. This is why projects need bleed, for example, so that a cutting variance of 1mm won’t leave unsightly unprinted edges showing.
The most obvious and common manufacturing variances can be seen with specialty options such as embossing, debossing, hot foil stamping, and spot UV.
It is difficult to get a spot UV, for example, placed exactly on top of your cover title without some pieces showing a millimeter of variance. It’s not extremely noticeable, but next time you are in your local bookstore take a close look at a few of the titles. Can you spot the variance?