Why is pixeSpacing[2] not equal to spacing between slices from DICOM tag (0018, 0088) ?

Beginner Questions
1

Hi, everyone
I read a series of CT DICOM images using ITK. And I get the pixel spacing using the method like
pixelSpacing = image->GetSpacing();

but the pixelSpacing[2] != spacing between slices from DICOM tag(0018, 0088).
pixelSpacing[2] should be the z direction of CT images.

I want to know the reason. Thanks.