Answer: You can’t compare apples to oranges
When it comes to paper making, three variables come into play: weight, caliper (or thickness) and smoothness. Think of it like a 3-ring circus, and the mills can only control two of those 3 rings.
Paper is made by using a specific amount of fiber and fillers to create a sheet of a fixed weight. To achieve a certain level of smoothness, the fibers get highly compressed and smoothed by calendering (think: ironing). This process causes the paper to lose thickness and therefore stiffness, but the weight of the fiber is still the same.
Other mills concentrate on the thickness of the sheets. Paper board, C1S, C2S and reply cards are usually called out in points (pt.), meaning the caliper and stiffness of the sheet are guaranteed, which is essential when it comes to projects with postal requirements or packaging. The weight of an 8 pt. sheet can vary from the equivalent of 68 lb. to 85 lb. Cover.