Thickness is determined by the mat specification. The authorisation body (BS, IEC, ASTM, etc) determine what the thickness of the matting has to be to meet the paticular matting specification rating.
The thickness therefore does not necessarily mean the thicker the matting the higher the protection will be in working voltage terms. SO PLEASE DOUBLE CHECK THE RATING REQUIRED FOR YOUR EQUIPMENT AND NOT THE THICKNESS OF THE PRODUCT BEING OFFERED.