As all ammunition ages it "off gasses" the chemicals and solvents used to manufacture the propellant. Different countries of origin use different chemicals in the production of their ammunition.
About 10 years ago when I was spending an enormous amount of time reloading 5.56 mm I bought about 60 rds of the Chinese "norinco" 5.56mm to check against my reloads. I'm running a 14.5" Armalite M15A4 and the accuracy difference is less than a 1/2 inch increase at 100 yards. Since 95% of my AR shooting is "run & gun" at under 150 yards I have noticed minimal problems with accuracy.