It is but we have an excess, that's the nature of Metabisulphite it's seems more than the Molecular weight, and it acts that way in use.

I think the whole Bisulphite/Metabisuphite properties are poorly defined and explained, there is lot of free SO2 when you buy fresh Metabisulphite, not included in the MW.

All the work I can find is into how Metabisulphite is used in practice, and the small difference in the US grade "Bisulphite". I know from practical experience there's a difference.