As others have the gap between 500 and 50 is huge, and perhaps even huger between 50 and top 5.

I was in a top 50 US guild, which (I'm guessing, I don't remember exactly) would put it near middle-ish top 500 world. In a top 500 guild the reason it might take 400 tries to down a heroic boss is largely a cascade of individual mistakes by various players. Raider #3 misses a heal, Raider #17 stands in the puddle of shit 0.5 seconds too long, Raider #8 messes up kiting. It was not entirely uncommon, at least in my guild, for a raider to show up with less than a night's worth of potions, to have one or two unenchanted pieces of gear, or even non-ideal specs.

In a top 50 guild, the individual skill of each player is much higher. Imagine the best player you've ever played with in your server first guild, and then clone them 25 times. That is a top 50 guild. Heroic attempts are usually (and to some degree I'm guessing here) ruined by "systematic effects": the interaction of two or more people's jobs going against one another, maybe because of a slight flaw in the strat or its execution. An MT healer accidentally pulls aggro from the Hunter kiting something with a big AOE heal, or whatever. Individual mistakes are almost non-existent though, and when they happen everyone is expected to learn from it and not repeat it. Other crap like I mentioned above with regards to raiding mats, specs, and gear would absolutely not be tolerated at this level, and would likely get you gkicked (and believe me, they check on a regular basis especially if you under-perform).

Now take your best raider, and multiply their talent and skill by 10, add tons of "free time" that they can play, and clone them 25 times. Now you have a top 5 guild. Heroic attempts are failed because they are done:

(a) while trying to establish a working strat (b) with largely heroic 5 man blue gear (c) on what is usually the hardest version of a boss (initial release) (d) with shitty server reliability (lovely first week of a new patch)

The human factor, however, is gone. If somebody effs up once, they get chewed out hard. If they eff up twice, or somebody else makes the same mistake, shit gets real and a new raider comes in. Also at this level everyone is expected to have multiple geared alts and know how to play them at the level of a main, to serve in whatever role is needed (cue 25 Druid boss kills).