When using a grid, the start line must not necessarily start on a grid dot. The end line, however, is moved in grid steps.
You can avoid such grid effects by pressing the Alt key when inserting space.
In column/row diagrams you can insert or remove space only in one column/row at a time. While all relevant model items are moved when space is inserted or removed, only the object occurrences are moved when the column/row size is changed.