it could be fake, but you should be able to tell if it comes close to the breaking strength, if so, then maybe not
dyneema and spectra, the two competing fibers, are both supplied to mills which make various different products.
better mills have better treatments and coatings for the base fiber, and better or worse quality control when spinning the fibers into strands and braids. UHMWPE, (both Spectra and Dyneema) , will exhibit an initial stretching or set, and any braid or lay (twist) will also have a "set" before it settles into the final stiffness. better mills pre-set their pre-spun fibers, and finished braids if the specs call for a non- stretch application. This extra treatment and pre-setting is probably what makes one product more popular than another.
All is not lost if you have a roll of the not so top shelf stuff. Simply spool off what you need for the next string, and one end off to something and tie a handle to grip with on the other end and do your own pre-stretching/setting before making the string. you should be able to feel the "bounce" and be able to work it some in tension, just under the breaking strength, before looping it on your jig. A little destructive testing on sample will tell you if the roll you have is a keeper.