It depends if it comes from pallets, if that's the case it could very well be from binders and fillers.
Also with raws it can just be residues left behind after the hormone has been synthesized and filtered..
Last but not least can sometimes be suspect to be moisture..
I have literally witnessed Trenbolone come out absolutely flawless after brewing, then left undisturbed for several days only to find it "foggy".. reheated it and swirled with the glass rod, and again several days later returning back to the same state, then filtering it all over again, heating and swirling it once again coming out absolutely clear with a beautiful amber color, only to fog again..
IMO, since this is not as common, it doesn't happen that frequently I believe a lot has to do with the raw material having leftover residues..