You are running a benchmark in a virtualized environment with no guarantees about access to the underlying hardware from one moment to the next.

It's also the smallest instance type. At least if you had chosen the largest you'd possibly have the entire box. With the smallest instance type however anyone with a larger instance than you is going to steal cycles and I/O away the second they have any load.

What makes people think benchmarking on virtualized hardware is at all worthwhile? (Serious question) It's like writing a blog post about which way the wind was blowing for the last five minutes. This is all nonsense.