78. LED Selection & Failure

Question.2

A company uses Vishay TLDR6400 (5 mm Red LED) on all boards at 20 mA.
DigiKey part- Vishay TLDR6400  
All LEDs have the same part number, same circuit, same supply.
Yet some LEDs appear brighter and some are slightly dimmer.
What is the most likely reason?
 

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