Here is an explanation I just emailed, and I think it will answer a lot of questions for many... sorry, its a little technical, as I have designed deli papers with logos, and wrapping papers... same basic process as fabric design...
When the printing plates are made to print on something like this (wrapping paper, fabrics, deli paper with logos) the art is done at precise measurements to wrap the plate around a specific printing press. There are hundreds of cylinder sizes... But the art is repeating along the length of the fabric, not the width, hence the need to find your repeat along the selvedge, ensuring that the "purple flower" your looking for a repeat of is the exact same measure UP from your selvedge on both. If the fabric is run thru the printingpress crooked, trying to cut a repeat via the WOF will shift nearly every time.