miniDV - Same resolution but lost vertical space in 16:9 widescreen?
Okay, after some reasoning and research I think I understand what happens. 

Basically, the sensor is fixed in size, but the lens is too. Which means the only way to get a wider sample to fit the image to 16:9 is to crop down the image on the sensor to about 720x405 pixels, each pixel catching a sample of ratio 64:45 (SAR).

The image is then stretched vertically to fit the whole 576 lines of the PAL standard. This means the original 5:4 footage captures has the same horizontal FOV as the footage captured in 4:3 mode, but a lower one vertically, stretched up to fit the whole 576 lines. This allows for the recorded media to then be stretched back into 16:9 by the media player. 

This last step (the vertical stretching done by the camcorder) is still something that sounds really puzzling to me, but after analyzing some more footage, it is rather obvious that it is a digital upscaling and not an analog capture of the samples, since a lot of the digital noise is stretched vertically too. 

It is an interesting way to get around the hardware limitations of the camcorder, but in the end not really worth it, considering the loss in quality.