The only thing I can think of is maybe you were planning to use the 2 yard cut like a center panel of your back then build it out with borders but 1 yard wouldn't be enough for what you would need to build out to the needed size given the top's measurements. In my experience most fabric requirements given for a quilt exceed what is actually needed. Maybe you were thinking you could use some of the leftovers in your back or maybe you had something in your stash in mind to make up the difference? Is it possible you bought excess of one of the top fabrics with just this purpose in mind?