Why the difference in percentage? Both attackers are Swordsmen with Combat II and defender is Town Guard.
This happens probably because of merging different stuff with ACO (which is in RevDCM). It could be S&D, but also anything else that affects combat odds.
You can see that defender has different strength (8.32 and 8.68) although the percentage breakdown is the same in both cases (and neither one of them matches the final strength - it should be 10.2). S&D modifier (i.e. "xx% from surrounding units") is not present at all.
ACO mod is coded very...ahem...badly. Sorry to say that, but that source code is very difficult to modify (and merge with other stuff), and when someone does it, it is very likely that a bug will occur.
I don't think I can help here, since S&D by itself is working OK (at least for me). Afforess should check whether all the new stuff he added is considered properly by ACO.