Yes, each cloth has its own characteristics as do the canvases, and nothing is exactly sewn the same. During the fittings, though, the fittings can be adjusted towards what you want to feel, and take advantage of the new characteristics. The second fitting, putting on the sleeves and collar may change the body fit somewhat. Hopefully, not drastically, which would have to be sorted out again. This reminds me of my grandfather. So much is about the customer and very little about the tailor. After all! It will be the customer wearing the garment. The end result should be a work of art with all the details you want. So the "very little about the tailor" is the arrangement of all the details, lines, shaping, etc. into a work of art. Some customers are good, sometimes better, at this to.
Exactly. I think the point that was in my mind when I wrote the post above was that, even though a particular jacket, say, fits like a glove, in the bespoke world that is no guarantee that the next one will fit the same way. Which is what you are saying. It's not really possible to say at any point that you have your fit "dialed in" to the point where you no longer need to pay attention.
Each new commission is a new journey. The starting point might not be the same as it once was, but it is a journey all the same.