Why do some LED's produce more light than others at the same voltage/current?

Interesting question. Since they are both GaN types, I would tend to guess that it is more about the phosphors used and the resulting distribution of the energy working with the human eye's sensitivities that lead to the bulk of the differences. Or else about the viewing angles (which you snipped in the bud.) Finally, it might be due to a backside reflector of some kind in the brighter one.

It would be hard to believe that a factor of 6 in efficiency would occur on GaN processing alone.

Looks like those two data sheets were dead copies of each other except for the difference in mcd and part number. Perhaps Don K. will teach us about what is going on here...

Jon