First of all , welcome to WPcentral.
My dear friend , the camera is 8.7 , that .7 is being used to create pictures that look good on both 16:9 and 4:3 aspect ratios , other cameras kill 4:3 pictures (popular for shooting tall buildings.)
This camera instead gives you a perfect 8mp 4:3 picture and a 7.1mp 16:9 picture.
Iphone 5 by default uses 4:3 ratio .
Lumia 920 uses 19:6 by default
If the iphone 5 wanted to shoot a 16:9 picture it has to delete lots of data of its 4:3 ratio picture .
This why the sensor is 8.7 to allow that easy conversion
here's a picture to easy your mind ...
View attachment 25742
and here's a 16:9 ratio picture by the Lumia 920
https://skydrive.live.com/pagenotfounderror
and here's a 4:3 ratio picture by the same phone .
https://skydrive.live.com/pagenotfounderror
I hope you got the idea ...
Cheers .
@nabkawe5