Why 70 percent alcohol are more potent than 80 percent alcohol?

You have been misinformed. The more alcohol, the more "potent" the drink, period. It may be that the particular drink mixed with 140 proof (70%) uses more liquor than the one made with 160 proof (80%). Either would be plenty potent, and would require a lot of mixer to mask the flavor of that much alcohol.

If you meant 70 and 80 proof, the same remarks would apply except for the mixer.