clewPretty sure the S15/S15R has a lower output because it uses a different kind of battery.
EDIT: That is to say because it can use AA, they both have Li-Ion batteries inside. Also based on previous versions of these flashlights the S15 has had lower output see selfbuilt's S10-L2 ( http://www.candlepowerforums.com/vb/showthread.php?369769 ) S15-L2 ( http://www.candlepowerforums.com/vb/showthread.php?367813 ) reviews to see max output is lower in the S15.
Edit 2: Reading more of selfbuilt's review, I think maybe in part due to having to have AA battery compatibility... the circuitry in the S15 just has plain lower maximum output (at least for the period before thermal cutoff happens and output is lowered 50%... notice that is 4.5 minutes for the S10R and 15 minutes for the S15R) .