There are many knds of silk in fabric store, synthetic(poly), mixed poly/silk, poly/cotton, silk/cotton, silk/linen etc.
And they will sell any of these as just "silk" with different price.
As I am in Thailand, I can tell you that silk may come with different grades. Either came from different worms or having different processes (hand draw vs machine draw for example). But I think the effects are smaller than real/blend/synthetic products.