Other people referred to it, but to state it clearly, TVs are measured from the corners of the picture while monitors are measured from the corners of the tube. Monitor manufacturers now list both numbers.

Exactly why that's the case, I don't know. My guess is that the viewable area on a TV goes much further into the corners of the tube than on a monitor (as huge corner convergence issues on a TV are commonplace but would be significant flaws on monitors) and that the dimension difference between tubes and viewable areas on a TV is insignificant. In fact, it may be that the basic tube for a 15" TV and a 15" monitor is exactly the same, with more of the monitor's tube ``blacked out'' than the TV's. I haven't looked at a bare TV tube in a long time to verify that, though.