Question about film speed data

I understand the actual film speed is usually different than box speed and it has other variables such as how you develop it.

My question is. Why does Kodak's data sheet says, e.g. TMX100, ISO 100 and 200 is the same development time, and 2TMY or Tri X, 400 and 800 is the same development time. Until 2 or more stops then the time will be different. Why do they do that? Or its a vague reference point that doesn't mean anything much? They can't be the same?!

Well, the ISO speed is well defined and it's what's printed on the box, although it is obtained with specific developer(s). The "actual" film speed is whatever might that be, based on whatever definition.

Kodak doesn't provide a separate development time for 1 stop underexposure, because you can print such negatives with a harder paper/filter grade. Basically this means that you can even mix your exposures in the same roll and use the same development time. You wouldn't get +1 stop film speed increase anyway with somewhat increased development time. If you underexpose by 2 stops, then you need to push process, in order to get reasonable density and print with a reasonable grade. Push processing will also give somewhat sharper toe, you'll need less exposure to get 0,1 density units above fog, so you'll get better shadow detail.

All of us have made mistakes when metering a scene (especially when blindly following TTL meters) and underexposure by 1 stop is quite common. These negatives will be somewhat tougher to print, but you'll get reasonably good results.

It is important to remember that adjusting your development time has almost no effect on shadow density - it only strongly affects contrast, and mostly in the highlights at that.

So when dealing with no more than a one stop "push", Kodak's testing indicates that there is enough latitude in the film to permit leaving the development time unadjusted, and thus leave the highlight contrast as is.

Thank you very much everybody, that's what I had in mind also but wasn't too sure. Thanks for making it very clear.