You're seeing the effect of "Table elimination" optimization (MySQL plan has 4 rows while MariaDB one has only 3). The MariaDB plan should be the better one as there is "less work to do". Both should return the same results though.
The feature is explained in detail here:
http://s.petrunia.net/blog/?p=58
and here:
If you want MariaDB to use the same plan as MySQL you can enforce this by disabling the table elimination optimization with:
SET optimizer_switch='table_elimination=off';