Nokia's approach of downsampling high-resolution output to 5MP images produces decent results in bright light but makes even more sense in low light, when images are captured at higher sensitivities.  In the downsampling process noise is being averaged out and the resulting image is generally cleaner with better fine detail.

The difference to more conventional smartphone cameras in evident in all areas of the frame. Check the green low-contrast areas or the resolution stripes, you'll see that the 1520 has a clear advantage over its rivals at higher ISOs.