Am I not? I will try to put it out as simple as that even you can understand it.
Okay. So you have these 2 bosses, that are after eachother in an instance. Are you following?
Like Horridon/Council, Durumu/Primordius etc. Still following?
If you check how many players killed each boss, comparing it with how many players killed the next boss you will know how many people that are still 'wiping' on a boss. This is of course not including pugs that for some reason registers on WoWprogress and guilds that faction/realm change/change guild name which can create duplicate guild. Say my guild is 6/12N and change server / disband. That is forever a guild stuck on 6/12
So, by checking this we can see how much of a 'brick wall' a boss is, how many % of the guilds that have access to a boss that is still wiping there. By extent, this also means how big the 'gap' is, since if boss 1 is very easy and boss 2 very hard, more guilds will reach that brick wall.
If you do this check for ToT normals and heroic on every boss and put it together. You will see that normal modes are far, far, far easier than heroics in comparison. The gap is a lot smaller in normals.
The 1 drawback of this is that we can not check how many guilds is actually wiping on Jin'Rokh. Though given how much of a 'loot pinata' Jin'Rokh is considered to be, I think we can logically claim this to be a non-important factor. Just as pugs killing 1/12N and pugs in general is a huge factor on the first normal mode bosses.
Add that all together and we get this.
Now these numbers are a bit old since I did this last week.
The two normal modes that stand out is Horridon, which I agree is way way way too hard.
Aswell as Durumu. Durumu is in my opinion not to hard for a normal mode boss, the success rate is still far higher there than on the heroic mode bosses.
Durumu could probably have switched places with Twins to make it a bit better progression, but Ji-Kun being a bit of a loot pinata aswell of course makes durumus success rate lower.