LCDs are not perfect. When fully off, they still leak some light through. When fully on, there is some transmission loss. A pixel technology with zero light leakage (because it has no backlight) can therefore show a wider range of intensities for each color channel, and thus a wider range of colors.
If you look at it from the perspective of 8-bit-per-pixel RGB, the theoretical range is 255 intensity levels per channel, but if LCDs lose the bottom 10% that leaves 230 available levels (theoretical 0-255 versus practical 25-255).
All of that said, I wonder how visible the difference is. This seems like something that would be easy to game by playing with the signal processing. It would be interesting to see results from double-blind testing, from an organization that doesn't depend on advertising dollars from the manufacturers in question. Hopefully prices on these eventually get reasonable enough for Consumer Reports to do some proper testing at some point.