Even the unlit kindle eink looks much more white with much blacker text (i.e. higher contrast) when viewed in direct sunlight, so internal lighting built into the Paperwhite should too.
Regarding a 25% increase, we must remember that eye sensitivity is non-linear (logarithmic), which must be taken into account when measuring perceived contrast differences. It cannot have improved contrast in all lighting conditions, because the internal lighting would be swamped by bright ambient or direct lighting, such as sunlight on the screen. In fact, the new capacitive touchscreen MUST detract slightly from the contrast (but perhaps not enough to be noticed by most people). Even the "microprinted light guide" must detract somewhat for some viewing conditions just because it is in the viewing path.