Why are there flux bins?

I was looking to purchase a cool white XM-L and am confused by the flux bins.

The prices at a particular site range from $8 to $11, with the higher prices corresponding to increasing flux from about 800 to 950 lumens at 3A.

Is this a result of variations in the production process? (It seems like it would make a lot more sense to pay a little extra for more lumens/watt)

Yep, it's variations in the production process. After each LED is made it's tested for brightness and tint, then put in its respective bin.

i am agree with you, but would you please help me know more details abbout the " variations " you mentioned?

Thanks for this!

Right... LED manufacturing isn't my forte, so hopefully someone will chime in if I say something wrong.

The way LEDs are made is that we start off with a wafer of some semiconductors, a common one is InGaN. This wafer then gets cut up into dice and phosphor gets deposited on it to convert the blue light produced by the bare die into white light.
* I doubt the wafer's composition/structure is 100% uniform (maybe more like 99.99% uniform), so you've got variations there.
* Any undesired elements will probably cause a reduction in efficiency (like uniformity though, there'll probably be hardly any impurities).
* The thickness of the phosphor layer would also come in to it, a thicker layer will result in a warmer and less efficient LED compared to a cooler LED made with the same materials. I suspect that they've only got a few different settings for cool and warm LEDs, then the rest is just up to chance.

There're probably more, but those are all I can think of for now.

http://www.madehow.com/Volume-1/Light-Emitting-Diode-LED.html
http://www.orboptronix.com/product_binning.html

Not everyone needs the highest output bin. Some of us are looking for certain CCT or CRI.