I am sure there are many factors which effect the width of fabric, especialy the origin of it manuifacture, quality of the cotton, number of pases through the dying/printing process. Some years ago I asked a friend who designed about the width because her fabric was coming thru at 40" as well as 41, 42 etc. She said they only assure the fabric is at least 40". If you measure all the pieces in a single line of fabric, you will find differences in the width. It depends on where the piece was manufactured. At one time the simpler patterns were produced in the USA while the more detailed from overseas.
Knowing the fabric has become more important now a days than ever before Read the label/end of the bolt and buy/work accordingly. I have even found some at 39". One just must account for this difference.