Both CRT monitors and CRT TVs, in the UK at least, are measured from the corners of the tube, rather than from the corners of the viewable area.

IIRC that used to be the case, but after getting slapped around by the Advertising Standards people, monitor manufacturers agreed to quote visible area. So it may just be that the "15in" monitor is older than the "13in" television -- old enough to be before the rules changed.

Peter