I have noticed that whilst I develop onto an iPad mini (1) that the designer is 1024 x 768
But when running it is only showing 1004 x 748
Designs are stretched to fit the 1004 width, so that views in the space 1004-1024 are truncated from view.
You shouldn't try to exactly match the device screen size as the actual size can change between different devices, different versions and different device states.
You should instead use anchors, auto scale and designer scripts to implement a flexible layout.
The 20 pixels are related to one of the bars.