This was the conclusion of consumerlab.com after several tests. The permissible deviation is defined by the European Pharmacopoeia at a maximum upper and lower limit of 10%.

In Finland, it was found that only one third of the tested products fell within a 20 % deviation or even contained nothing..
https://www.vitamindcouncil.org/blog/is-your-vitamin-d-supplement-defici...

More info:
- http://goedgezond.info/2016/02/vitamine-d-supplementen-getest-hoe-kies-j... (Dutch)
- http://newhope.com/supply-news-amp-analysis/problems-discovered-8-vitami...
- http://www.vitamindwiki.com/Average+vitamin+D+contained+only+33%25+of+wh...