there is essentially an infinite number of variations that can be made to the testosterone molecule. in addition they can create synthetics with active sites that are similar as a T derived steroid but are completely different in the rest of their composition.
every steroid has different effects and every person metabolizes steroids(and all drugs) differently. some can become androgen and then estrogen, some cant. people have individual reactions to all drugs.
additionally there are probably hundreds of russian, chinese, iranian etc steroids that the west doesnt know about or barely does. additionally there are precursors, "androgen like" products and non-anabolic performance enhancers.
there is a shitload of steroids. im sure UFC fighters alone have tested positive for 20 different variations.