Trapping is a concept not widely understood outside of the walls of the printer. The idea is that colors that touch sometimes need a little help getting there so that no white space can be seen between colors. Graphic Impressions maintains our equipment regularly so registration issues typically are a matter of distortion from pressure as the paper travels through the press. Paper makes a big difference.
Offset printing presses have many rollers that the paper must travel through. Each set of rollers has the potential to impact the original dimension of the paper, even if only slightly.
See examples below for a clearer understanding.
This image shows a leaf created with a green spot color (PMS) and Black ink. Where the two inks touch there is a very small overlap of color. Most times this isn't noticeable to the naked eye. What is noticeable is when there is no trap. A little goes a long way.
The samples below show how the same image might look from sheet to sheet without trapping.