Docker faster than Host?

Hi,
I’m running a benchmark to test IO runtime differences between Docker containers and its host, and I noticed something strange. I’ve performed random writes/reads.
The storage driver for the container is aufs.

If the file to be written/read is smaller or equals to 1GB, docker is faster than the host (otherwise, if the file is bigger, docker is slower).
Why do I get those results for small files?