http://www-01.ibm.com/support/docview.wss?uid=swg27024735 (http://www-01.ibm.com/support/docview.wss?uid=swg27024735)
http://www-01.ibm.com/support/docview.wss?uid=swg21590332 (http://www-01.ibm.com/support/docview.wss?uid=swg21590332)
In short, the OS/390 indexer, under an exceptionally rare condition has the potential to generate incorrect indexing values and/or some lines of the report not getting stored.
I can't say it any better than the doc above.
PM58481
IF FMID(H272841) THEN UK77812
IF FMID(H27284A) THEN UK77811
IF FMID(H272850) THEN UK77813