Lumia phones (at least high-end ones like the 920) have a sensor that supports multiple aspect ratios. The full resolution of the sensor on the Lumia 920 is 3552x2448, which works out to just about 8.7 megapixels, but that full resolution is never used.
When you set the camera app to take pictures in 4:3 aspect ratio, the full height of the sensor is used, for an image resolution of 3264x2448 (8 MP). With a 16:9 aspect ratio, however, the full width of the sensor is used, for an image resolution of 3552x1998 (7.1 MP).
The following image from the Lumia 920 imaging whitepaper illustrates this well:
This sort of sensor design dates back at least as far as the 2011 Nokia N9. The reason for it is to minimise the loss of resolution when switching between aspect ratios. With the 920, when you switch from 4:3 to 16:9, you lose about 11% resolution. Compare this to traditional 4:3 sensors where 16:9 is achieved by simply cropping the image, resulting in a 25% loss of resolution
To address the practical aspect of the question - to change the resolution back to what it was, simply change the aspect ratio. Both the Camera and Lumia Camera apps let you choose between 4:3 and 16:9. There is no other way to set the image resolution on the Lumia 920 (unlike the higher-end models with oversampling, like 1020 and 930).