Because nobody actually tests the Deltas on the testing shard. There's a handful of players that patch in Blight's files and play the game, but they can't be relied on for accurate testing data because there are many, MANY hardware + OS combinations out there that the small testing group couldn't hope to cover with their playtime.
You are the testing group since the
actual testing group is so miniscule in number.
If you encounter problems while playing, send a ticket in to
support with as much information as possible about the issues you encounter. They will ask you if they need any more information, and will attempt to resolve your gameplay issues.
I personally have not encountered any major choppiness (aside from when I first log in, but it clears up in minutes) with the latest update.