The quilting process 'draws up' the backing and batting- both need to be larger than the quilt top (often quilters require 3"-5" larger all the way around) to make sure you do not wind up short nearing the end and also to compensate for any (not quite even/straight) issue with the top. The excess batting/backing is trimmed away after the quilting is completed as the whole quilt is squared up before the binding. Also, it is very difficult to keep a top and backing perfectly lined up with each other as it is quilted. There are ways but it certainly adds a lot of labor to the project