To expand on what Larry mentioned.
I’m a 2013 user and I’ve run into this scenario quite a bit. Unfortunately, Project doesn’t necessarily handle your two scenarios exactly the same. When you first create the fixed 1 day duration task, the duration is now fixed. When you add the resource and provide a 16 hours estimate, project can’t change the duration. That’s fixed. It can’t update the hours, because you just entered that, so the only thing left to do is to change the assignment units as you’ve seen to 200%.
Once the resource is already on the task, it works very slightly different. If you change the hours, say to 32, the duration is fixed, so it can’t change that. You just entered the hours, so that can’t change. Which leaves the assignment units already sitting at 200%, which I’m assuming is already exceeding the resources Max Units. In this case, Project leaves the assignment units as is (200%) and puts the REAL assignment units in a field called Peak. The Peak field is what it will really take to accomplish 32 hours of work in 1 day. So the 400% is in Peak and the Assignment Units still shows 200%. The reality is that while Project always keeps that task formula in balance (duration = work / availability) it is really keeping it in balance with the Peak field, not the Assignment Units field.
Continuing this discussion a bit, this is one of the scenarios that Project will NOT fix with resource leveling. It’s a task configuration problem and as such, you the PM have to manually fix the task by either adding 3 days duration, reducing work to 8 hours, or what ever.
So how do you find these scenarios in your schedule? As Larry indicated, create a Task Usage view with columns for Max units (will only show on the resource row), Assignment Units, and Peak. Anytime Assignment units exceed Max Units or Peak exceed Assignment Units, you probably have an issue to resolve.
Hope that helps