Me-thinks he's leaving a white border around the print, cuts the window opening in the overmat a tad larger than the image itself, signs his name in the white border area of the print, then positions the print in the window (with photo corners on the backing mat) so it looks like a traditionally dry mounted print.
This way if the mat gets damaged it can be easily replaced and there's no chance the edge of the actual image can be chipped off, like with a print that's been trimmed right into the image.
Murray