Frommycarolinahome is a blog that I have followed for a few years. If you read the article, she states that she would usually send the top back to the maker to fix. But in this case, it was a charity quilt that she was quilting for free. Putting darts in the borders to counter extra fullness, is a radical fix and in this case, worked very well. For those who deal with donated tops, it is not always time or cost effective to properly correct every error.
Too much fullness in the border is often the result when the border has not been cut to fit or is on the bottom while sewing.