The "para-cord" has a natural stretch to it, weaved like it is you get more stretch than a single strand of the same total length do to the tightening of the knots if you will.
And as far as strength, knotting weakens rope and cord.

"Any knot in rope will weaken it. Knots with the greatest load applied to very sharp bends are weakest. The strongest knots spread the load gradually over some distance before there is a grip."
–  Charles Warner, A Fresh Approach to Knotting, 23