Mostly cautious. Why risk things over a couple of weeks?
Let's round the accutane half-life to 24 hours to make calculations simple. Remember at a 24 hour half life you will have more in your system at any given time that with a lower half life.
If you take 1 unit per day, you will never have more than 2 units in your system at any time.
7 days from your last dose you will have just 1/64 (.015625) units in your system. After just 2 weeks you'll be down to 1/8192 (.000122) units. This doesn't sound like much for synthetic Vitamin A, but without substantial research why would you not wait a couple of weeks to provide some cushion?
My unscientific opinion is that they are worried that even after the accutane leaves your system they are worried about your body restoring its natural skin oil supply. And since the drug probably hasn't been tested on pregnant women (I did not search pubmed before posting this!), why in the world would you add another variable into pregnancy?
Caution in this case is not a bad thing. It stems from skepticism in regards to drug use.