The only difference between ephedrine and psuedo-ephedrine is the stereochemistry of the alpha carbon (the one with the OH attached). The chirality of this center is lost when the OH is removed to produce methamphetamine. The critical chiral center is that of the beta carbon. (psuedo)ephedrine with the S designation at the beta carbon will yield (S)-Methamphetamine(the good stuff). However, the Thai meth factories would most likely begin with a racemic mixture of (psuedo)ephedrine. While there are paths to converting the isomers(oxidation at beta position, amination, reduction), it is not likely that the factories put much effort into the purification of their product. Therefore, the end composition of their product is based on the enantiomeric purity of their ephedrine or psuedo-ephedrine source.