Help with HTML/CSS Assessment (failed)

Hi everyone. Just got the results of my HTML/CSS Assessment and I am a little puzzled. It said that I missed making the form 600px wide when the screen is >= 600px wide. I remember doing this spec requirement by indicating the grid layout for columns 600px, since the columns would dictate the form’s width. I also tested it on my browser for accuracy.

Anyway, would like to clarify if the testing tool is specifically looking for an explicit width spec of 600px for it to mark it as correct or I am doing something wrong?

Please see the screenshot below.