Because it's true.
The shorthand for the cream gene is Ig which comes from inhibitor of gold.
In simple terms there are only two feather pigment colors - black and red. White is the absence of either. Different gene switches affect the distribution and intensity of pigments along the feather. Barring turns color on and off along the feather as it grows and affects both colors. Cream or IG only inhibits the expression of gold i.e. red.
Good source for learning basics of chicken colour genetics is Sigrid van Dort's book which can be ordered but not read online. A cheat sheet of sorts can be ffound here.
Http://sellers.kippenjungle.NFL/page.html
I think you are thinking of autosomal red (Ar) which is not completely affected by the silver gene (S)..another gold inhibitor.
Hope this helps