Sometimes the online version of product grows in content offerings while its print sibling remains bound by its print limits:
Important Details: The American Psychological Association (APA) announced
that it will discontinue print publication of its Psychological Abstracts
Index after the December 2006 issue. The decision was attributed to a
sharp decline in print subscriptions (from a high of 9,500 to less than
300); the wide availability of the online product; and the coverage gap
between the products - the print included only about 32 percent of
materials covered in the online version.
From http://www.library.yale.edu/~llicense/ListArchives/0607/msg00174.html