The number and concentration of chemical impurities is the difference. In addition, analytical methods used to determine concentration may be not adequate and vary from one manufacturing site to another.
I can vouch for that. I occasionally take a generic Lasix and I can take one and it will work immediately and strongly. Then sometimes I’ll take one and get nothing. Same bottle, different results.
We deal with notified bodies every year at work.
There are specific guidelines laid down in standards from different organizations such ISO, ISTA, ASTM and the like.
With ASTM standards multiple laboratories in different locations determine if the tests are valid. So it shouldn’t matter if you manufacture in Switzerland or Swaziland.
You must conform to the standards.
If the company is not doing so it should be caught by auditors.
It sounds like both were not doing their jobs.