It's not the calc field. (Add Date of Orientation and you'll get the same thing.) It's a tableau limit: maximim of 6 columns.
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Actually the default is 6 , the max is 16. You can change the default in Analysis-Table Layout-Advanced and change the Maximum levels of column labels
Just wanted to highlight this VERY IMPORTANT comment! The maximum number of columns is, as stated above, much higher than 6, but you have to change the Table Layout as described above.
To add to what Alex said, I believe the max of 16 will be increasing in an upcoming version. There was an unsupported hack of editing the XML in the workbook to exceed 16, but if that change is actually on the horizon, that's a welcome change!