Some of you guys need to re read the friggin post.
That being said, its not damage to the carpet if they are keeping the chairs on the mats.. If they don't then you have mulitple problems. But I see it all the time when chairs never leave the mat. It is soiling and to an extend could be linked with filtration soiling in my opinion. You kinda have a double edged effect going on.
First of all of the oily soils around the carpet get tracked on to the mat just like hard surface but there is no where for that soiling to go but right back on the carpet. You also have all of the shoe dirt being loosened up on the mats. Add to that the very very fine plastic debris from the bottom of the casters and you have three sources of soiling that get tracked right off onto carpet at the edges.
The other side of that is the fact that all carpet fades to an extent. things that change the rate of fade will include light sources.. (even florecent lighting), the rate of off gassing and air flow around the carpet and foot traffic. The mats provide protection against direct light exposure, they trap air flow and moisture, and they prevent foot traffic.
Combine these scenarios with the build up of soil which over time can oxidize slowly and discolor carpets, and you have a unsightly lines and sometimes will come up if its just soiling or sometimes because the rate of fade is different will never catch up.