First off...welcome to the Quilting Board!
I ran into this too when I really started looking at patterns, could't figure out the math. There can be many variables depending on the pattern. Each pattern designer may calculate yardage differently and may be giving estimates to leave enough for mistakes. However size of the pieces of the blocks will really determine how much yardage you might need.
To give you an example; on another thread we are discussing a mystery quilt that will finish 82 x 82. The estimated yardage is 13 yards but the designer has a lot of smaller blocks and pieces. As strange as it sounds the bigger the block, the less yardage you will probably need.
As other's have said, if you post the pattern (or a link to the pattern) someone should be able to help a little more.