Why is there so often a delay on kills being announced?

DISCLAIMER: My video contains language inappropriate for those under the age of 17 without a parent or legal guardian!!

http://plays.tv/video/56f3675ecdce768720/rito-why-does-this-happen-so-often-?from=user

In this video, I kill Yi, and the Announcer declares it like 10 seconds later. This happens a lot, and in teamfights I sometimes cannot follow how many enemy champions are alive at any given time because of the delays.

Thanks for looking into it!