as a grain is a measure of weight. At least, what I've read.
You are absolutely correct and yes, it can be confusing. What "they" have done, is make a close proximity "Field" conversion so it's handier to load. That will allow us to load without the use of a scale. .....
BP and replacement propellants, are very forgiving and not much is lost by being a few grains off either way. Where in smokless, being off a littls, can be dangerous. ......
Be Safe !!!