They are isomers of each other, but there are two stereocentres and the one they differ is reduced to H so no longer acts as a stereocenter (there are only 2 isomers of methamphetamine not 4 like ephedrine). At opne point I was obsessed with finding the answer to this question, just out of curiosity and suprisingly there is not a lot of info out there on there internet. Most info is unreliable user report etc.