That because 1620X1080 has an aspect ration of 3:2 which is the same as most SLR cameras. If you want an aspect ration of 16:9 then you will HAVE to crop to that aspect ration in Lightroom before exporting. Another option is to use Photoshop to warp/stretch the image but that will mess up the photo.
Thank you for your reply Bob. Fully understand. However, my DSLR (Canon 5D Mk111) gives me the option to shoot in 16:9 according to the Camera settings. Having set the camera to 16:9 and after editing in LR, when I attempt to Export as 1920x1080, I only get 1620x1080 - as you say 3:2.
Super frustrating as I can't afford to crop the images.
I too have the 5Dmark3. If you set the aspect ration Lightroom should have cropped it this way. When you set the aspect ration in the camera to 16:9 it doesn't change the view in the viewfinder and it is still 3:2 so you have to estimate what is going to be removed on the top and bottom when you crop in Lightroom. It does show the crop in live view. If you are shooting RAW it still records the full 3:2 and you can recover this in Canon DPP but Lightroom unfortunately has been purposefully programmed to not recover this info for this particular camera. It does for other cameras. I don't think it is ever a good idea to set the aspect in the Mark 3 if you are going to be using lightroom. Unfortunately you can not do what you want to do in Lightroom with these photos.