It's been over a year since I used 2007. If someone with more relevant and timely information reads this, please chime in.
I recall having the exact same problem. Turns out that the settings for your WBS task affect how these fields behave when you complete a task.
You can make a task type fixed duration, fixed work, or fixed units. If these settings are wrong, you will get this behavior. You'll also need to look at all of the different sets of fields (i.e. % completed, % work completed, duration, actual X, etc.) to figure out if you've got it right.
I believe the task type needs to be "fixed units" to do what you are expecting it to do. But beware, changing this value after you've entered data will give weird results. Also, check out the "effort driven" flag as it will also impact how Project behaves.
If you're not using Project server, you might want to archive your project and experiment with these values.
I'm pretty sure this is the most painful piece of software I've every worked with, just for this reason.
As I said, someone else chime in if you have more/better information.
Let us know the results, please.
