Why do some abilities finish the cast after champion death and some not? Why did Riot decide so?

For example why does {{champion:245}} 's Q return after he dies but {{champion:101}} 's W doesn't activate after he died but the animation already started. There are other examples too for example {{champion:91}} 's W not returning after he dies. I'm not ranting or anything just curious why does Riot decide that some abilities can finish after champion death and some not.
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