Historically, ratios were used for peering between ISPs of the same type. When one backbone peers with another backbone, 1:1 ratio can be a simple proxy for equal value. (ISPs liked simplicity in the old days because it made the Internet cheaper than the complex telco X.25/ATM/SONET networks with their complex tariffs and settlement.) When a content provider peers with a broadband ISP, they can exchange equal value without exchanging equal traffic.