Some LED lead frames are coated in a silver plating to increase the reflectance of light leaving the die, but this leads to easy sulfuration. Sulfuration is a chemical reaction between sulfur and silver.
Sulfur corrodes silver. Sulfur penetrates the LED and discolors the silver coating on the lead frame over time. It can cause the wire delamination and influence low luminous intensity. In extreme cases, it will cause a complete failure of your LED.
Therefore, some LEDs do not contain any silver coated lead frame and use anti-sulfur components.
To learn more anti-sulfur details, please check out our forum post entitled What is the Benefit of Using Anti-Sulfur Components?