Actually, you are looking at the effect and making an assumption about the cause. A "weekly cap" is actually much harder to code, as are diminishing returns and whatnot, and I just don't see them doing it. What is FAR FAR more likely is that a vast majority of the people that run raids on a weekly basis do it the day they reset, as such you have a higher proportion getting them that day. Combined with that is also the possibility that people have multiple toons, and run them every day of the week. As such, there is a minor tendency towards the beginning of the week, as say I have 14 max toons, I run each raid twice each day, on 2 toons. If on tuesday I get the invincible drop, I will not be killing LK for Invincible again the rest of the week. it is a minor tendency, but it is still a tendency toward drops the beginning of the week. It might have a minor impact, but I would put my money on it being a vast majority run them the day it resets. I know when I was still running regularly, that's when I did it, so on the off week when I forgot or had a conflict, I still had 6 days to do it, whereas if you wait till Monday and have a conflict, you miss that week.
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If they are 1% drops, which i actually think they are a little higher, and you ONLY ran the bosses that dropped those three, then yes, 1 in a million. But as you run more bosses that have a chance to drop, the odds of getting at least 3 mounts goes up quite a bit. And like someone else said, even with really low odds SOMEONE is bound to have it happen, because there are so many people doing it so often. I mean, the other day I got both the mounts to drop of Malygos in EoE in a single kill. While the odds of that happening THAT specific kill are small, the odds of it happening at some point in my play time isn't that small, and the odds of it happening at some point to some player is almost a guarantee.