SimpleITK Direction Matrix Behavior

Hello, I am having trouble understanding the behavior of the calculation of SimpleITK’s direction matrix. I sort a series in increasing z value based on ImagePositionPatient, but when i pass those files into ImageSeriesReader to create an Image, the direction matrix is (1,0,0,0,1,0,0,0,-1), which does not make sense to me. I am new working in this field, so any help would be appreciated.

Best,
Colby