Phone Camera: More Megapixels isn't always better

A 13 megapixel smartphone camera might sound great, but a 8 megapixel shooter will do better due to the following reasons.

sensor

 This is the part that captures the light. The sensor is essentially the "film" material of a digital camera. No light, no photo.
Light enters through the camera lens, then passes to the camera sensor, which receives the information and translates it into an electronic signal. From there, the image processor creates the image and fine-tunes it to correct for a typical set of photographic flaws, like noise.
The size of the image sensor is extremely important. In general, the larger the sensor, the larger your pixels, and the larger the pixels, the more light you can collect. The more light you can catch, the better your image can be.
Larger sensors are the reason that 8 megapixels from a digital camera is better than those 8 megapixels from a smartphone camera. You get roughly the same number of pixels, but the pixels on the digital camera get to be larger, and therefore let in more light. More light equals less-noisy images and greater dynamic range.

 Image processor

Most modern high-end smartphone CPUs have dedicated graphics processors built into the chip, which, being hardware-accelerated and not just software-dependent, can quickly render images like photos, videos, and games without overtaxing the main application processor. 

The user

The user's ability to snap under perfect exposure, i.e the direction of light, composure, also determines how good the outcome of pictures can be. Recently, some phones have the ability to detect faces, smiles e.t.c which makes pictures to look better.