If you have sub pixel mapping checked (in advanced menu) RT will produce a darker version than the normal render.
The problem was also mentioned here:
http://forums.chaosgroup.com/showthread.php?78010-Dark-RT-CPU-RENDER
Just to confirm this is behavior still happens in 3.00.06 for 2014 and 2015, but the bug fix of unchecking S.P.M. still works ![]()
Thought the problem and solution might be more visible here.