Illustrator :: Resizing Objects Using Transform Window Is Broken?

resizing objects by changing the width and height values in the Transform window, while keeping the proportions of the object? When I try this in CS6, the proportions of the object gets totally botched.
 
This is what I do:

1. Draw a straight line that is rotated some 30-40°.
2. Select the line and click in the W: or H: fields in the Transform window.
3. Make sure that the "Constrain Width and Height Proportions" button is selected.
4. Press and hold the up arrow key on the keyboard.
5. Watch how the line is rotated as it is scaled.
 
I notice that while I'm pressing the up or down keys, the number in the selected width or height field is incremented nicely (21 mm, 22 mm, 23 mm, 24 mm, etc.), but when I release the key, it is adjusted to a seemingly random number (for example 24 mm -> 24.168 mm). If I switch between pressing up and down for some time, the adjustment can be even more random (24 mm -> 58,944 mm).
 
I recently upgraded from CS3 where this type of scaling worked flawlessly. I'm using a MacBook Pro with OS X 10.7.5.