Using preset you are producing uniform images with person head in defined position within image and same looking margins around it. It may happens that original image is too small or person face is too large for used preset and cropping box is oversized and extends outside the image bounds.
By default the program shrinks the cropping box to fit within image borders while keeping the centre point of the face in the position defined by preset.
If you want to keep faces same sized and don’t care for visual background quality, for example if you making green screen photos with following background removal, you may want to keep the face size by expanding image bounds.
Outside areas will be filled with colour the same as the color of top-right point of the cropped rectangle. If you shot your models over plain colour background (like green screen for future background removal) you’ll get quite good results suitable for further work.