My gut tells me that all fabric of identical specification is not the same...if corners can be cut, and it can be made more cheaply, someone will do it. Moreover, different mills will have different levels of QC in place, and some probably have none.
How do you differentiate between the good, the bad, and the ugly? Danged if I know.
And this: the fabric you buy today from a given vendor is almost certainly not the same thing that was on the shelf a year ago.