This is made clear in Prisoner of Azkaban, where Harry has to defer catching the Snitch until Gryffindor has accumulated enough of a lead, otherwise Slytherin wins the cup:
Slytherin was leading the tournament by exactly two hundred points. This meant (as Wood constantly reminded his team) that they needed to win the match by more than that amount to win the Cup. It also meant that the burden of winning fell largely on Harry, because capturing the Snitch was worth one hundred and fifty points.
"So you must catch it only if we're more than fifty points up," Wood told Harry constantly. "Only if we're more than fifty points up, Harry, or we win the match but lose the Cup.
Prisoner of Azkaban Chapter 15: "The Quidditch Final"
Fred and George also have a detailed discussion of point differentials in the aftermath of Gryffindor's loss to Hufflepuff, considering situations in which Gryffindor still has a chance to win the Cup:
"It's not over yet," said Fred. "We lost by a hundred points, right? So if Hufflepuff loses to Ravenclaw and we beat Ravenclaw and Slytherin -."
"Hufflepuff'll have to lose by at least two hundred points," said George.
"But if they beat Ravenclaw..."
"No way, Ravenclaw is too good. But if Slytherin loses against Hufflepuff..."
"It all depends on the points - a margin of a hundred either way."
Prisoner of Azkaban Chapter 9: "Grim Defeat"
This is an even better example than I'd first imagined, because Prisoner of Azkaban is also a year with a tied win/loss record; Gryffindor and Slytherin each have two wins and a loss:
- Gryffindor wins against Slytherin and Ravenclaw, and loses against Hufflepuff (because of Dementor interference)
- Slytherin defeats Ravenclaw and Hufflepuff, but loses against Gryffindor