Everyone who says this is obviously someone who just went full retard... Hasn't it occurred to you that there are actually amazing players out there who do crazy things? Saying that they're bad isn't an excuse. Mabye the assassin dps is just better than the other dps out there..... Moron.
Clearly, PvP rank doesn't require much game knowledge.
Assassins aren't terribad, but assuming the mara isn't the one going full 'tard, they typically will be at the top in DPS & definitely not under sins. If this guy was at the top of the charts in his 'hardcore' raid guild, then the rest of the dps was pretty sub-par & they're about as hardcore as my 7yo son's leveling guild in WoW. It's not about amazing players doing crazy things (though that certainly helps), it's just the toolset limitations on the class and basic math comprehension.
Sins were never routinely beating maras, hasn't changed since the patch. This guild = baddies being bad imo.