It's not about "5/8", it's about the 1/2" ID, and maintaining it throughout one's rig. That way, you have a minimal pressure drop at the block's outlets. In this case, there was a pressure drop caused by the small barb diameter, and another from the transition from 1/2" ID tubing to the 3/8" ID of the barb, which also affects the flow geometry.
We're still talking about minor things, so that's probably why it's a "5002", and not a "6000".
