some of this is just supplement 101: the amount of active ingredient *always* varies, that is normal. there are acceptable degrees of variability
however, it is of concern, when we find only half the amount, or worse, twice the expected amount!
a few problems with their article: who are the manufacturers? i understand they don't want to tell. but you know, Dollar Tree makes vitmin D now. and so does walgreens. and so does vitacost. and so does carlon's. this is a great reminder to know your manufacturer
and finally, I highly recommend skipping pills altogether and getting oil-based vit D3.