It is the honest scoreboard. Commission is visible, but shortfall also captures the price drift while you waited, the bid-ask-spread you paid, the market-impact you caused and the opportunity cost of unfilled shares.
Algos branded "implementation shortfall" or "arrival price" trade more urgently at the start, front-loading execution to limit drift risk.
Example: decision price 40.00 for 100,000 shares. You fill 80,000 at an average 40.12 and never get the last 20,000, which later trade at 40.60. Executed cost is 80,000 × 0.12 = $9,600; opportunity cost is 20,000 × 0.60 = $12,000. Total shortfall $21,600, against a commission of maybe $500.
Related: arrival-price, slippage