Because we embroider on various fabrics, which all have a natural stretch, there will always be some pull on your fabric as the hoop moves back and forth and the stitches are formed. This causes distortion and unevenness. The amount of pulling depends on a number of factors, such as the fabric you are using and how well you hoop it, the type of stabilizer you applied, the size of your design, the type of fill stitch and underlay stitching you are using. All of these together effect the outcome of your embroidery. During embroidery application, stitches pull the fabric in with the direction of the stitches. This can result in gaps in your stitch out.
Also, in the case of an area of fill, the stitching pushes out at a right angle to the line of stitching, so the embroidery will end up further out then what it looks like on your screen.
To compensate for the push and pull effects of machine embroidery for various types of fabrics and stitch types, you need to develop a certain level of digitizing experience.