Printers cannot print right to the edge of a paper sheet. To create that effect, the printer must use a sheet which is larger than the document size, printing beyond the edge of the finished size, then cutting the paper down to the final size. It’s impossible to stack the sheets exactly on top of each other to make the cut. The extended image insures that no white space will appear when the work is trimmed and finished.
It is the designer’s responsibility to set the bleed correctly. 1/8” (.125”) is standard.