Why 360 video resolution is 16:9? [closed]

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I have a 360 video (Filmed using Garmin Virb).

I tried to extract some frames from it, to do some processing, but apparently, the aspect ratio is 16:9.

I expected it to be 2:1, to match a normal equirectangular, but it isn't.

What is the correct way to change it to an equirectangular image?

I'm worried that simply stretching it will have side-effects that I'm unaware of.