Default behaviour:

Each new sale order line is recorded with the default sequence (ex. 10). Ordering of lines in sale order form then is done by sequence, id. When any sale order line is moved to another position, every existing line's sequence is rewriting to preserve the current order. Adding a new line, as a next step, records it with default sequence (ex. 10), pushing it right after the first existing line. If added lines are multiple, then reordering by hand so they are positioned in the end is a time consuming.

New behaviour:

Each sale order line is recorded with sequential sequence. There won't be any default sequence. Each new line receive a sequence resulting from last existing one. This way adding lines leaves them always at the bottom of the list.