In the default configuration of a new user account, a single tab will take you from the Defer Until field to the Due field.
In this case, it sounds like you’ve enabled tabbing to “All controls” in System Peferences (under Keyboard Preferences > Shortcuts):
When you do this, the Tab key also gives you access to the +1 day / week / month buttons between the two text fields—which, naturally, means it takes more tabs to get from one field to the other.
As the System Preference screenshot indicates, you can quickly toggle between those two settings (without taking your hands from the keyboard) by pressing Control+F7.
