Yep, as a few above me already stated:

1) Better unit yields

2) Statistically better stability(for a given number of units)

There's a reason why certain CPUs will overclock better than others of the same bin, and why the manufacturer does not sell them at a higher bin, for a CPU manufacturer it all depends on yields. For a device manufacturer, it all depends on what speed will give the best stability for a large number of units, while not compromising performance.