In multiple choice questions with multiple answers allowed, the square around the radio button indicates the last choice selected. Excellent user experience, an improvement over the standard format.
In multiple choice questions with only one answer, as well as in true/false questions, the square around the radio button serves no purpose whatsoever, or does it?, is non-standard and can be confusing. Not a great UX either, no need for an outline square in these cases.
The first sentense should read "In multiple choice questions with multiple answers allowed, the square around the checkbox indicates..."
For information only. Outline boxes used on radio buttons and checkboxes, as seen in a Google UX course on Coursera:
I'm not sure how the accessibility borders relate to the scrollbar issue?