Why does the Lumia 1020 have a 41MP sensor?

Can anyone explain to me why the Lumia 1020 has a 41 Megapixel sensor, when the largest picture it can output is 38MP, and the oversampled 5MP picture has a 7 pixel oversampling per pixel (which means the sensor made use of 35MP worth of sensor)?

Is it to do with the 38MP and 34MP pictures (in 4:3 and 16:9 respectively) requiring slightly different pixel positions on the sensor meaning that overall a total 41MP sensor size is required?