Why does Riot sometimes knowingly buff champions/items past the point of balance and then not nerf them until after a while? (This isn't some passive aggression/anger-fueled question. I genuinely want to know why they do it OR if I'm misunderstanding their balance ideals).
To sell skins. Prime examples: 
Tbh at this point I’m entirely convinced they are just taking champions randomly and rolling dice. 1-3 are nerfs, how badly is determined by the number rolled 4-6 are buffs. Again how good determined by the number rolled.
And if you get a skin you roll a d12 instead, and it’s always a buff.
This might be hard for me to explain so I googled it and I am about to copy/paste the exact definition that is widely accepted, this may be too difficult for us idiots to understand and only the Rioters balance team gods will fully grasp this difficult concept, so, without further-ado I present to you enlightenment:
money /ˈmʌni/ noun a current medium of exchange in the form of coins and banknotes; coins and banknotes collectively. "I counted the money before putting it in my wallet" synonyms: cash, hard cash, ready money; More FORMAL sums of money. plural noun: moneys; plural noun: monies "a statement of all moneys paid into and out of the account" the assets, property, and resources owned by someone or something; wealth. "the college is very short of money" synonyms: wealth, riches, fortune, affluence, assets, liquid assets, resources, substance, means, deep pockets, prosperity "she married him for his money"
Because there are so many elos and factors at this point that doing anything is just a crapshoot on whether people will accept it as a good idea or not.
We have reached a point that balancing and making everyone happy isn’t possible, not that it ever was, so they just balance in an attempt to make the most people happy as possible, the people who like the balance just play and the people upset with it bitch until it is tipped in their favor which leads the new people to bitch as we get this never ending cycle of buffs, nerfs, praise, qq, etc.
My advice, ignore the balance, ignore the boards, play what you like and you will advance
Theres two reasons:
Their "ideals" seem very mixed.
In one breath they release a busted version of the champion item and slowly bring it down. Akali But then in another breath they say we'd rather have X champion/item weak and slowly bring it up... Akali (even though it was quick compared to some other champions)
So personally I don't think they have a "plan" or "ideal" perse that is universal or the standard they follow.
One important factor to consider champion wise is favoritism. Yasuo CANNOT be weak as this would hurt Riot financially, but Udyr can because his fan base is nothing in comparison. With that in mind if Yasuo needs balance then Overshoot and nerf slowly applies. For Udyr it's break his legs and slowly bring him up.
So for more popular champions, 
(Surprisingly). Will not remain in a weak state for long and when "adjusted" will likely be over shot knowingly.
Additionally are skins, champions will more than likely receive buffs out of the blue for a temporary boost in sales.
From a different angle you have new release. Most of the times the champions will be released over tuned due to the learning curve associated with them.
rework was released fairly weak (compared to most releases) because most players could easily pick up that kit.
were heavily over tuned due to their learning curves.
Of course none of this is official, but it stands true for the most part with maybe a few exceptions.
Game is pretty good tbh the boards are kinda crazy
