We do try, but it is physically impossible to get a particular pantone colour looking exactly the same on the entire range of promotional merchandise.
The reason is that different items are made from varying materials and are printed differently on the various machines. These machines in turn require their own ink systems and different ink systems have different types of pigmentation.
Another issue is that colour perception depends under what light source you view something. We all know how something on a computer screen looks different depending on what angle you look at it. A printer who is printing promotional merchandise could match items under a light source, but under a different light source they may not match and might even look completely different from each other.
Tricky stuff, sometimes!