Is it defined in firmware or hardware?
Why is NTSC output 720x480 and not the standard 720x525? After overscan, the average NTSC TV will only display 656x448 on a Pi. But if the standard is used, the average TV will output 656x483 after overscan and blank data removal.
That's an 8% increase in scanlines. Which is pretty significant when dealing with such low resolutions.
If the issue is because the scanlines need to be a multiple of 16, why not have it be 720x512? That'll give us a visible resolution of 656x476 on the average TV, which is still a 7% increase in scanlines.
All of this also applies to PAL. The Pi puts out 720x576 in PAL, but PAL standard is 720x625.