Your question is based on the assumption that crystalization works to selectively isolate methamphetamine and that no other molecules appear in the crystal lattice. If that were true, all meth would be of equal potency. I suspect the assumption is at least in part incorrect. Pure methamphetamine crystals would no doubt form under ideal circumstances. I doubt very much that the cicumstances most meth crystals are grow in are ideal, or that most meth crystals are pure methamphetamine. Most contain some by products and some unreduced iodomethamphetamine.
If you recrystalize the meth crystals several times, they will become cleaner, purer, and stronger. You will notice after each recrystalization that the mother liquor has the odor of something other than meth. It will have trash that is not included in the crystal lattice this time, but was the last time that crystal formed. Slow crystal formation results in purer crystals than rapid crystal formation. Neither assures you that the crystal is "pure" methamphetamine. An improvement, yes. Pure, probably not.
It may also be that when one grows meth crystals, these crystals may be intermixed with crystals of unreduced iodometh, or other such impostors. How can you be sure that the multitude of crystal formations one sees are all meth?
Interestingly enough, the meth from SWIG's first 36 hour reflux was not recrystalized at all. It was used as it was, after evaporation and acetone flashing. (Time was short, and the demons were hungry.) The potency that was noticed was no doubt the result of there being fewer byproducts in the mix of what was in the pile. It had nothing to do with recrystalization. The product of his second long reflux was recrystalized once, in a rather fast fashion rather than a long, slow growth mode. Not what SWIG would call artful crystals, but definitely potent. Stuff that spinouts for the unwary are made of. Not to say that a long, slow recrystalization would have improved the product, it would. The product was not completely clean after one crystalization. It left noticable trash on glass, and he has made it before that left virtually no traces after being vaporized in glass. The long reflux allowed more complete reduction, and a greater concentration of methamphetamine among the molecules in the crystals formed. The crystals were of greater potentcy than those which contained more impurities. The stuff from which the crystals were grown was purer and the crystals themselves purer than those obtained from less completely reduced batches.
Mostly harmless