“Fill between slices” effect fills between parallel slices (along any of the 3 axes). See detailed description of the algorithm here.
Any editing operation in the Segment Editor will resample all the segments to the common labelmap geometry. If you have just imported a segment from another segmentation that had different geometry (higher resolution, different origin, or axis directions) then the segment will change upon the first editing operation. Segments are not immediately forced to the common image geometry to allow lossless moving/copying of segments between segmentations.