Why do photos from the same digital camera have different megapixels?

Photos from a nikon 12.3 mgp camera can have 1mpg or up to 6 mgp? Why doesn't every photo have 12.3 mgp?

It depends on how compressible the image is.

If the camera was set to take different resolution photos, the mega-pixels will be different. And if the image is cropped or edited the resolution will be reduced.