explain plan anomoly

Oracle 11.2.0.4 SE-1 and 12.1.0.2 SE-2, on Win 2012

Two databases, two servers.  11.2 is prod, 12.1 is test, recently upgraded from 11.2.  Still in acceptance testing before upgrading prod.  Question is not about the fact that the two plans are different. I expected that.  But some specifics about the differences are very puzzling.

Here's the setup.  I start with a sql script named 'explain_prod.sql' to run and report the plan:  Note the dynamic naming of the spool file to include the instance name.

delete from plan_tablewhere statement_id = 'MINE';commit;set trimsp on lines 256COL operation   FORMAT A30COL options     FORMAT A15