The paper discusses this, they say that 90%-110% is the usual range which is allowed when the drug is sold new. The FDA can set limits on a per-drug basis, and there are some where they require 95-105%, but 90-110% is the most common. (C.f. https://www.fda.gov/ohrms/dockets/ac/06/briefing/2006-4228B1...)
Several of the drugs they tested were > 100%, so it seems reasonable to assume that a lot of this is due to variation in manufacturing rather than to the drug degrading.
Several of the drugs they tested were > 100%, so it seems reasonable to assume that a lot of this is due to variation in manufacturing rather than to the drug degrading.