This developer is diluted for use so the actual sulfite content is 5.0 g/l. There is some argument whether the iodide is necessary or even useful in a developer of this type.
This developer is a variant of the Beutler formula which uses 10 g of sulfite in the working solution. Crawley found that this could be reduced to 5.o g/l. He was a careful and thorough experimenter. I would speculate that he found this value to be the minimum which would produce good results.