2. Reasons for the differences 

The dimensions of the current ISO paper format sizes were first mentioned  by a German Professor named George Lichtenburg in the mid 17^th century. It was scientifically derived  whereby the pages have a constant height to width ratio of the square root of 2 (i.e. 1:1.414) no matter how many times it is folded or cut in half along  its shorter side.

This is known as the aspect ratio and the specific dimensions in this case is considered to be aesthetically proportionate and pleasing .The consistency of these proportions can be confirmed by dividing the height with the width  for all ISO formats. This is illustrated in table #1.

Table # 1 The aspect ratio  

In addition, in the ISO system the weight of the paper was simply the weight of one square meter of the paper and was expressed in grams per square metre, or its grammage, irrespective of whether it was cover stock, text paper or art card.

The Americans only began to standardize their paper formats in 1920.

It appears that the committee responsible for the standardization was staffed mainly by businessmen from the printing and paper industries.

Being practical people, it appears they based their recommendations mainly on commercial considerations to minimize wastages of existing paper stock inventory , and compromised to ensure that no major changes or costs would be required on their existing paper manufacturing and processing  machines  to effect compliance with the new standards. It appears they came up with about five major categories of paper each with a different basic sheet size as listed in table #2.

Table # 2 Basic US sheet sizes 

In addition, calculating the weight of the paper was relatively more complicated .The weight  was expressed in pounds and was the total weight of one ream of paper (500shts) of a specific basic sheet size. This means that a 60 lb. text stock is not necessarily equivalent to a 60 lb. cover stock.