Here's what he had to say:

The sink rate range is because different size lines sink at different rates. The lightest line in a series will sink at the slowest rate, the heaviest at the fastest rate. We list the sink rate of the fastest sinking part of the line, which is the belly in most cases. Sink rate testing is done in freshwater. Only a very small percentage of sinking lines sold are used in saltwater. I don't know how much the higher density of saltwater will affect sink rate, but would guess 10% or so.
(emphasis added)

It was purely a curiosity question, but I thought it was interestin', figured ya'll might, too.

Thanks to tailingloop for Mr. Richard's name/email in reply to my original post (shame on me for forgetting to say thanks in that thread).