All pharmaceutical preparations, from outside of the US and from inside our borders,  Will have varying degrees of potency  from "batch to batch".
Sometimes less sometimes more, but  variations are to be expected in any drug. This is even true of supplements, too.  The degree bhow much  this can vary  is monitored by the FDA  here in the states.  But there is not a monitor for every pill that's produced.  
 The reason being, drugs or supplements or even food can be affected by many factors.  The slightest variation of ingredients can sometimes have huge effect.Environmental factors can also be considered as well as the people or machines  preparing them.  Storage,  exposure to temperatures and light, can affect the potency.  
 Then you have to take into consideration your own biology.   Tolerance, hydration, sleep, diet, other substances being drugs or supplements and again food,  can  influence  your response