for rppm explanation. Trusting it since it's a blue post.
From what I read in that post, each rppm item has it's own rate that is then multiplied by your melee/ranged/haste rate, based on which is higher, and by the last time it activated divided by 60 sec. The very bottom gives that exact math. I read nothing about each individual event dividing up the chance to proc it.
My understanding is this: (I'll use the numbers in the post): Windsong 2(ppm) * 1.22(haste) * 1.4(time since last chance) / 60 (sec per min) = 5.693%. That 5.693% is the chance each event can proc Windsong again.
Even the changes in http://us.battle.net/wow/en/forum/topic/8197741003
state nothing about each event dividing up the proc chance.
And yellow ^.^ Blizz stated multiple times in their blue posts that it WAS to make it more random. They felt raiders needed something "more" to liven up raiding.
The old system was based on Weapon Speed, which would cause inconsistencies across classes due to varying uses. This system eliminated that Weapon Speed calculation and purely took Haste as a factor. The proc chance per event, as long as each class has the same amount of haste, is now the same instead of wildly different.
If I missed a section or misinterpreted a part then let me know. I've reread both posts about 3-5 times and couldn't find any Blue posts other than those two I posted above.
Edit: First post was about Weapon Enchants, as that was the first development for RPPM system, which was later integrated into ToT trinkets.