Why L2 cache access time increases in this benchmark?

I ran this benchmark several times and I got the same results. What this benchmark is doing is reading a big array with different steps. The Y-axis is the Access Time (the time that took to read the array), and the X-axis is the step size.

As we can see there is a big jump when the step size grows larger than 2^15 bytes (32KB, the size of my L1 cache) and when it grows larger than 2^20 bytes (1 MB, the size of my L2 cache). However, the is another jump at 2^18 bytes, and I don't know why. I've run the benchmark several times and I always get this jump.

My CPU is an Intel i7-4770K, 3.5GHz

Thank you very much!