sort curves by z value

i'm trying to sort frame of curves (4 curves) according to their z value. Basically i want the highest curve (highest Z value) and the lowest curve (lowest Z value). the curves are out of order due to their construction method(s). So i figured the best way was to sort by the z value. However when doing so, I can get the bottom curve just fine, however, when i try to get the top curve, it starts dropping some of the curves for some unknown reason. I have tried everything I can think of t get the last curve in the list after they have been sorted but am unable to do so.... 

have i missed something?