What makes completion latency various in Fio benchmark with NVMe SSD?

I'm trying to figure out the completion latency of fio benchmark with NVMe SSD. I made following fio script to test benchmark in fio. I used following options. rw=read, ioengine=sync, direct=1 So, I thought there's not much things to make completion times different. However, the result wasn't what I thought. The result is 1th : 11us to 99.99th : 111 us. Synchronous read make no outstanding IOs, so all I/Os are processed sequentially, And direct option can bypass the buffer in OS. I thought most of latencies are same. Any ideas for this result?