Echo giving 10% CD + 10% CD instead of 20% CD?
On Echo it says 10% CD and then passive: additional 10% CD. Why didnt they just put 20% CD in the first place, without the passive?
1 Comments
Because 10% of those 20% come from a unique passive effect called "Haste".
Now, in case you didn't know, "unique passives" with the same name don't stack, so if you build 1 item with a unique passive named "Haste" you won't get the 10% CDR again if you build another item a the unique passive named "Haste".
The item affect here are 
Archangel's Staff, 
Hextech GLP-800, 
Luden's Echo, and perhaps most importantly the one item all 3 of those items are build out of, 
Lost Chapter.
All those 3 items have an insanely cost effective build path and are all very strong power spikes on their own, especially and are a very deadly combo on paper, so giving all those items a passive effect to make them less cost effective when build together makes a lot of sense if your goal is to give AP champions a strong power spike for early game but then encourage them to build lategame oriented items instead of just stacking these single power spike items.