First, there are 7000 grains in a pound. There should be a certain grain weight for the first 30 feet per the AFTMA standard.
I know you've seen it but here is the info: http://www.common-cents.info/aftma.pdf
or here:
http://www.rodbuildingforum.com/inde...=0&#entry55735

Weight isn't the issue of floating/sinking. It is the density of the line and the displacement it has. If it is less dense than the water, it will float. More dense, it will sink.

If the first 30 feet of a flyline is 500 grains, it isn't a 9 weight according to the AFTMA standard. They may be referring to another length or characteristic.