I do understand that the mods can't be everywhere and see everything; and if a post gets reported to them, of course that's the post they're going to look at--first. I wouldn't expect the mods to read through the entire preceding ten pages and do an in-depth analysis of all the participants who are at fault and to what degree. However, looking at a single post in a vacuum, outside the context of the thread that precipitated it, doesn't seem like the most accurate method, either. And when the mod has the power to publicly chastise, gag, ban, lock a thread, etc., it seems important that they exercise these powers carefully, or people will start to feel like there is "something else going on."
Perhaps, as cass2591 said, people need to be more proactive about reporting things. But, sometimes it's difficult because it's not one particular post that is horrible, but rather the "trend" of the thread. The last time I reported something, the post I chose to hit "report to moderator" on was not, on the surface, so terrible, but I felt it was the pinnacle of an offensive turn the thread was taking. I tried to convey this in the accompanying note to the mods, but perhaps I didn't do a good job of that. I was really hoping the mod would read back over the preceding few posts--not pages, just a few posts--and see the same trend I had seen, and at least say something about it publicly. But to my knowledge nothing was said, and I must admit that the thread seemed to "right itself" after a bit, which is great.
I guess my point is just--I was hoping the mods did look over at least a few preceding posts, and cass2591 indicates they don't. Which is good to know, but makes it more difficult for me to decide when to report, and what to report. Also, I'm curious if this might vary by mod--with some reading just the reported post, and others reading the ones right before it--and even within the same mod depending on how busy they are. Perhaps that leads to the inconsistencies that people have noticed, which could be interpreted by some as favoritism?