Why are the shipping rates returned by the shipping carrier different for English (US) and English (Canada)?

Shipping rates are being charged different because of the different Weight units being determined by the culture. The Weight Unit for English (US) is LBS and the Weight Unit for English (Canada) is KGS. So when 1.5 is entered as a weight, it is read by Pressero either based on LBS or KGS. The culture settings will determine which Weight Unit to use and depending on what rate you are using to ship the item, you could set the culture to avoid any rate mismatch. 
Rate example below:
Culture - English (US)
Weight Unit - LBS
Weight - 1.5
Rate - 15.08
Culture  - English (Canada)
Weight Unit - KGS 
Weight - 1.5
Rate - 12.98
Notice the calculated rate is different. Both weight values are the same but the Unit of Measure is different for US (LBS) and Canada (KGS).
Note - In the above example, the Shipping To and From address is same for both.