when using DFSORT or any smart sort program
for <duplicate> keys the order is unpredictable,
the workaround for DFSORT is to use OPTION EQUALS
which will for <duplicate> keys preserve the original relative sequence
speak to Your support to find out the defaults for each product, and You will find the why for the different behavior.
Talk to IBM. If you are migrating to DFSORT, the DFSORT team will be happy to assist with questions that arise.
It looks like CA-Sort can define symbols. So can DFSORT. Have a look at SYMNAMES in the documentation, and SYMNOUT which shows the generated symbol table. (I hope that ONE and TWO are only examples for this post...).
All the DFSORT manuals are available online.
1.12 is somewhat dated, you may want to check with IBM about support (or your boss should want to).