Low Impact Factors Explained
The reason for the drop in rank for A&A and ApJ comes from an error in the procedure used to count the number of citations to the papers published in these two journals.
The paper by A. Heck, "WRONG IMPACT!", published in the EAS Newsletter, Issue 26, December 2003, explains what happened.
It seems that in the late 1990s when ISI started to use shorter abbreviations for some major journals, they continued to count references to the long names for A&A and ApJ, but not the new shorter versions.