I may have found one reason why the Rogue with 7/51/13 provides such a low combopoint-generation, if this is the used configuration (I took it from .svn/trunk/melee.txt):
# Combat Rogue (Fist, Serrated Blades)
# Not fair to use the same stats since Combat Rogue needs less expertise.
# Trade expertise for ivalue-equal amount of attack power
# Throw in a pair of Paladins....... Eventually these static parms will be replaced with real actors.
I would recommend a Glyph of Sinister Strike instead a Glyph of Blade Flurry. That should boost the CP/Sec, but the difference between 15/51/5 and 7/51/13 is still too high, as Ratak already stated. I don't think that a missing SS-Glyph is the only reason for the difference between the two combat specs:
Edit: The difference is 90,1 combo pionts over the duration of the run. Rogue_07_51_13 had 116.2 sinister strikes during that period. Unfortunately I have failed in finding the crit rate of the sinister strikes. The only value I've found was 51.6% but it is stated as "crit per execute" and I have to admit that I am not aware of it's meaning. But from my experience this crit value can be achieved in 25man raids. So by calculating 51.6% as crit rate for sinister strike this leads to ~60 critical sinister strikes. A SS-Glyph provides a 50% chance on critical sinister strikes to award an additional combo point. So using a SS-Glyph would boost the combo points from 96.9 to ~126.9. Still missing 60.1 combo points. Accounting the 20 combo points Rogue_15_51_05 gains from ruthlessness, the total difference between the two specs should be more closer to 187 : 167. So all in all Rogue_07_51_13 is missing somewhere around ~40.1 combo points. If there is a mistake in this calculation please point at it. The only difference between the two specs is ruthlessness, so the combo point generation deficit of 7/51/13 should only be the amount of the benefit ruthlessness gives.
One other thing: I glanced at the "Gains" for Rogue_18_51_02 and it appears that Adrenalin Rush isn't used as well for the 18/51/2-setup.
The difference would be that 15/51/5 performs more finishers, and thus gets more ruthlessness procs.