Think about both weak and strong defense teams as pairs.
Your alliance has 4 teams, all togheter add 1500 points.
Weak teams would be worth lets say 200 points
Strong teams would be worth lets say 550 points
The opponent alliance would have 4 Strong teams, every team worth 375 points.
Now, a pair of 2 teams is worth the same in both alliances (Weak + Strong teams = 200 + 550 = 750 points) vs (Strong + Strong teams = 375 * 2 = 750 points)
The difference is that your pair Weak + Strong can be defeated with 5 bad heroes and 5 good heroes while the pair Strong + Strong must be defeated with 10 good heroes. So you will run out of good heroes faster than your opponents.