When offsetting a face, if there is an intersection
between the face and the sketch plane, by default, it is this intersection
which is offset (rather than the projection of the face edges).
In this
case, if you want to offset the projection of the face edges, you can modify
the offset as explained in this task.
Double-click the offset in the specification tree (Offset
node) or on the sketch.
The Offset Definition dialog box is displayed to let you
modify the offset.
Click in the Object to offset field and select
the new sketcher element you want to offset.
In the Offset value field, type 20mm.
Choose Project and offset from the Offset
mode field.
Click OK to validate.
The offset is modified.