Two Netflix Enter a Mailbox...

Strange phenomenon of recent weeks... Every time I send back two Netflix movies at the same time, date, moment and mail slot, one gets there first and the other "arrives" the next day. How is it in the finely honed mailing system that is Netflix does this happen? Why does my conspiracy antenna go up and I feel like they hold on to one for an extra day? It has happened 3 times in the last 3 weeks. I wonder if this happens to anyone else? Keep you posted.