MariaDB is executing the query incorrectly - why?

Question

MariaDB

I have MariaDB version 5.5.35a
and this magic happens:
SELECT with FROM table WHERE a=7 and b=3;
result:
id
123
234
345
SELECT with FROM table WHERE a=7 and b=3 and c=100;
result:
id
100
i.e. the result 100 should be in the first result, but it's not there.
why? how to fix?