One commercially available set of Scrum poker cards use a sequence similar to the Fibonacci sequence:
0, ½, 1, 2, 3, 5, 8, 13, 20, 40, 100, and optionally a ? (unsure) and a coffee cup (I need a break)
As for why, it's to emphasise that difficulty does not increase linearly, it usually increases exponentially (hence the approximation of Fibonacci sequence).
Also see this post for other ideas as to the choice of this sequence.