Why did the US choose 173 grain 7.62 and Nato 146 grain bullet which is better

I may be wrong, but the 173 was the sniper/target weight bullet. Also, I think the Springfield  1903 and M1 Garand 30 caliber bullet was 173 grains for a while also

Cartridge, 7.62mm, Match, M118
Ballistics
Velocity 2550+-3ft/sec at 78 feet
Pressure 50,000 psi, max. avg.
Accuracy 3.5" mean radius, max. avg. at 600 yards

Cartridge 390 grs approx.
case 190-20 grs
bullet 175.5 - 3.0 grs.
propellant WC846, 44 grs./IMR4895, 42 grs.

IDENTIFICATION "Match" or "NM"

Cartridge, 7.62mm, NATO, Ball, M80
Ballistics
Velocity 2750+- 30 ft/sec at 78 feet
Pressure 50,000psi, max avg.
Accuracy 5" mean radius max avg. at 600 yards

Cartridge 393-31 grs.
case 190-20 grs.
Propellant WC856, 46 grs./IMR8138M 41.5 grs./IMR4475 41 grs