The Weight rating gives you a basic idea, the flex number is much more precise. To determine the flex number, you would put something under each end of the pole, and hang a 10-pound weight in the middle. I have a good spread of flex numbers 13.4, 13.9, 14.4, 15.0, and 15.5. All have the same weight rating, but depending on wind conditions, how fast I'm running that day, etc. There is a relation between the flex number and the weight rating .
Some people ONLY look at the flex number.