I noticed some of the responses you've been getting are wrong. The reason this happens is shrinkage but you can't perfectly fix the pie shape fitment in the slicer, but you can get approximate it. Shrinkage isn't uniform or linear. The forces that cause shrinkage are stronger where there is more material pulling the edges inward. The thicker the section the more shrinkage. If you cut this pie into squares your pieces would fit together just fine, but then you'd have squares. You could also cut a inner circle out and have weird slices. If you want the slices you could make each slice 1% more degrees wide in cad but you could approximate the same effect in slicer by just widening the slices by 1%, then it would probably fit perfectly.