Michel,
As I understand it, a large part of the problem with the BW is that two backing sheets are pulled through the loom under high tension. These are then stitched (almost) together and then parted into "A" and "B" rolls by a shearing blade.
It is these same rollers that are the motive force through the entire process, so the tension is maintained within the roll.
When a section of carpet is removed from the roll to be fitted, this tension starts to gradually relax (because it is more loosely rolled) causing the length of the section cut off to start reducing. When the carpet is laid out by the carpet fitter, without the friction within a roll, the relaxation accelerates.
Thus, even without the additional influence of atmospheric changes, if it was left laid out in the room for a couple of days without restraint, it would reduce much further in length. These is the reasons that it is recommended that it be acclimatised in the room prior to fitting and then not tension-fitted.
Even after several years in a room, if removed from the grippers, (e.g. for decorating) for a week or so, it can be imposible to stretch back.
