The problem is… The autogenerated key is always in the form [auth][year] irrespective of the preferences setting. This makes the preferences setting of little use.
these are imported through the medline/webinterface
7-10 are imported through [auth][year], the others using [authshort][shortyear]. The preference is changed in Options->Preferences->Bibtex-Key-Generator and saved. IMHO this has been working flawlessly since long.