Thanks for the info David. Can I just pin you down on what you mean by 'NTSC TV'?
o do you mean an old analogue NTSC 525 lines, 60Hz TV that has a composite input?
o or an 'NTSC' computer monitor? (that perhaps has an NTSC tuner in that used to allow reception of NTSC signals, but you're using the composite input)
o or a HD-Ready or Full-HD TV with composite input? (this would not be NTSC, nor PAL nor anything, as in the digital domain there are no such things
Here in the UK a HD-Ready TV has a native screen resolution of 720 lines. You can feed 625 lines into it (now actually termed 575 lines in the industry) and it will upscale that to 720 lines but still be standard definition. Or, if you feed 1080 lines (i.e. full high definition) into it that will be downscaled to 720 lines, so you will lose some of the full HD quality.
A Full-HD TV has a native screen resolution of 1080 lines. You can feed 625 lines into it and it will upscale that to 1080 lines but still be standard definition. Or, if you feed 1080 lines (i.e. full high definition) into it then you'll get full HD quality.
Some of these HDTVs can accept 525 lines too.
Now.... from your experiments it would seem to me that your NTSC TV is either the computer monitor or HD-Ready/Full-HD TV. I say this, as being in the industry, with respect I find it difficult to believe that an old analogue TV would lock to many of the resolutions you mention. But it'll be interesting to hear your reply to this.