Hola Paloma, or ciao...
That is because 1920x1080 will only be shown if your project has a 16:9 aspect ratio
Your project is set to 4:5 which is a resolution of 1080x1350.
Although in a resolution expression the left value is always the horizontal aspect and the
right value the vertical aspect there is something else to consider.
Up to HD videos the convention was to use the shorter side of your video (which is usually
the vertical dimension) as the name giving component. Thus your export is 1080p.
Remember -> 4:5 !!!
To confuse everybody totally in the jungle of formats and bitrates they decided to make
everything sound even more impressive and chose to name new resolution formats after
their longer dimension (the horizontal axis).
So 4k e.g. is not ‘horizontal x 3840’ but rather ‘3840 x vertical’ - in a 16:9 aspect ratio
this would be 3840x1920.
That is why!