Zoe, do you generally use print or slide film? Common film speeds include ISO 25, 50, 64, 100, 160, 200, 400, 800, 1600, and 3200. Print film for the consumer is generally in the 100 to 800 range, although 1600 isn't all that uncommon anymore. ISO 64 is usually a slide film rating, like Kodachrome 64 Professional. I don't know that I've ever seen ISO 64 for print film, although you can probably get it from a professional photographic supply house. If you use print most- or all of the time, that's probably why you aren't familiar with 64.

I don't know why the 64, 100, and 200 settings would be identical. I haven't experimented with changing ISO settings for the same setup with my D70s yet. The only thing I could think to ask is, did you use the same shutter speed and aperture (f-stop) for all of them? If you let the camera choose those automatically, it would have compensated for the ISO rating you were using. Perhaps, the 400 setting was too sensitive for even the top shutter speed of the camera, given the light conditions, and so the photo came out washed out.