Cause as others have stated, numerical for macro are rounded up. So you can either have sub 2.0 which pretty much is irrelevant and 3.0 which is a DPS loss. Secondly, in the event that something crops up like needing to do Silence while in the 2-3 second wait your macro will require to fire the first skill again and wait 2-3 seconds again for the subsequent one.

So in the event you get a River of Blood proc, ADS cast High Voltage you press Blunt Arrow you will miss that last River of Blood proc and if casting another GCD skill and then it should be 3 seconds pass and a next tick will happen if that happens to proc River of Blood although in your scenario you may never know, you have effectively lost 2 River of Blood Bloodletters.

@Sterob
There is no wasted off GCD skill, that's just stupid talk. See it this way for every Bloodletter full CD with no River of Blood proc the later you press Bloodletter the lesser Bloodletter you get. Same applies for Misery's End.