Actually, it is just another area in which lamp technology is still ahead of LEDs. LED generate light based on the materials used and the amount. Light is emitted based on electrons 'falling into gaps or holes' on the materials used. Slightly different materials, slightly different gaps or holes, slightly different light generated. And anything can effect it really if you thing about it. It is like wallpaper, no two batches will ever be exactly the same.
Lamps age predictably and if you replace lamps at the same time in fixtures, they will look the same so long as the optic trains are the same. Tungsten lamps are even more predictable since they change very little if at all over the course of their life. They aren't really built on the microscopic level like LEDs are and thus the advantage.