Oracle Autotrace vs Explain
Autotrace in SQL Developer gets the plan from v$sql_plan, and also gets the stats from your session, does a delta of session stats before and after running the query.

Explain Plan asks the database what it THINKS the plan will be for your query.

Your co-worker is right, they can differ wildly, and you're better served to use AutoTrace or our new feature in 4 and higher where we show the cached plan (a drop down control on the explain plan button makes these available.)