To be more specific, LED manufacturers have not perfected the process of producing LEDs. The manufacturing equipment is not exact enough to produce emitters with specific efficiency. If they could, all cool white emitters would be R2 bin (or better).
In reality, they just try to make them as best they can, but the efficiency of individual emitters are distributed normally. In other words, they follow a normal model, AKA a Bell curve. It's like the distribution of people's heights. If we just consider females, let's say the average (mean) height is about 5' 3". That doesn't mean that all females are 5' 3". We expect some to be a bit taller or a bit shorter. A few will be really tall or really short. The R2 emitters are like the WNBA players and models of the emitter world. For every really efficient emitter (or tall woman), there are plenty of average and below average emitters (or average and short women).
So no matter what, there are always going to only be a few of the highest bin emitters available. Currently this is R2. They can't all be the best.
Hope everyone appreciates my brief midnight stats lesson... I miss being a teacher sometimes (but not often!)