You haven't provided enough code for us to know what exactly it is you are benchmarking. In particular, how is your data initialized?
I for one am suspicious that maybe each iteration is allocating a new set of arrays, which may be allocating more memory, which may be causing your benchmark to swap.
I'm also wondering if the initialization of your arrays is deterministic, or if their are random values in those arrays that might be making the sort done by List::Compare take longer in some instances.
