Team Member 17.7 - Why does gray shading show up on columns on a Time Sheet in Team Member 17.7?

When different people open their Timesheets in Team Member 17.7 in the Cloud, some columns show up shaded in gray.  Time can be entered into a column that is shaded.  The gray shaded columns may be different for different people, although the date range is the same.  We are just wondering what triggers the different shading on columns.