Why does MariaDB 10.1.2 blow away MySQL 5.6.22 in sysbench?
The MariaDB has a lot of optimizer features not found in mySQL (especially 5.5, but still compared to 5.6)

See this link for a comparison of optimiser features.

There is also a benchmark results page which shows MariaDB faster in a lot of tests.

So in the end MariaDB has more options when choosing an execution plan.