In the Set Page Boxes dialog, under Margin Controls, when you use the
Apply to: dropdown to switch among boxes, the top, bottom, left and right values do not update to the selected box. For instance, if I change the values for the TrimBox only, then select Bleed Box, the values I just entered display for BleedBox. however, the calculated Cropped Page size for the BleedBox correctly does not include the margins I entered under TrimBox.

I also found that if I choose CropBox again, which in my sample case has all zeroes in the margin fields, the values restore to zero. Then I can click on BleedBox and see the correct all zero values. Switching back to TrimBox locks the values in again.