There is a document on AIX Strength to Strength that might give you some information on the changes. My best guess is that it has to do with a libc rte change (C library Run Time Environment). If the system that was used to build a package uses include files that are newer that the rte you try to run a package on the application may fail.

This is one of the more common issues regarding binary portability of applications?

afaik: ML09 (AIX 5.1 didnt have TL, that started with AIX 5.2 aND 5.3) is the last maintaince level pack for AIX 5.1. Note that there is a final (including updates made after the ML09 release date) update. This has replaced all the individual updates for AIX 5.1 (same for AIX 4.3.3)