Just because two fabric are from the same line, its highly unlikely that the blank yardage they're printed on was woven and processed at the same time. Also, after printing, the fabric is sprayed with fixatives and starches while being pulled over large rollers, then thru a heat press which irons and folds it. This process alone, because its done under firm tension and affected by chemicals, can often 'stretch' the yardage somewhat. When you prewaah you're removing this 'stretch' and all the chemicals, along with allowing the natural shrink of the fibres.
Even knowing this I still prefer not to prewash my fabrics (unless the colour is a possible 'runner') because as some have already mentioned, once quilted into a sandwich it all shrinks pretty evenly anyway. Honestly, if you're quilting 10" apart you're not quilting, you're asking for trouble. Batting manufacturers might sprout that spin in their advertising and it will appeal to those who want the quilting part over quickly. But it really is a good example of 'just because it SAYS it can be quilted 10" apart, it doesn't mean you should'.