Does NetFlix count differently than I do

We have been happy Netflix subscribers for a long time.

We are members of the "4 DVDs" plan, meaning that we can get up to 4 DVDs at a time.

But, several times recently, we have had 5 DVDs at once.

At first, I thought maybe Netflix had lost count?

But it has happened more than once, and this time, when I visited my queue, I noticed that not only did the Queue page list 5 entries under "DVDs At Home", but there was a link:

Why do you have an extra DVD?"

Yes, that was my question exactly! So I clicked the link, and Netflix told me:

We have sent you an extra DVD rental from your Queue.

NOTE: After your extra DVD rental arrives, get your next movie by returning any two movies.

Hmmm. I am pleased that they provided the link, and I am pleased that, for whatever reason, they sometimes send us 5 DVDs at a time rather than 4, but I still don't understand why I have an extra DVD.

Anybody understand what's going on here and can explain it to me?