Hi sanelss,
Yep, your edit is correct. The RS-232 port on the roboRIO adheres to RS-232 standards, also known as TIA/EIA-232-F (Page 5 of the specifications document found
here). This means that a logic 0 is represented by a voltage between 3 and 15V and a logic 1 is represented by a voltage between -3 and -15V.
It looks like the Arduino is only able to output Serial TTL which is just TTL logic, not TIA/EIA-232-F logic. Sparkfun actually has a good article on the differences and a solution to this problem located
here.