We even experience temperatures differently on different days. I remember one hike where first night I was just barely warm in my WM Ultralite (older model) and the following night was toasty warm. The temperature was actually lower the 2nd night by a few degrees. But the first night I was absolutely worn out and not real well fed and hydrated and the 2nd night I had had a leisurely day.

That said, I think it is helpful to consider bag ratings along with a "personal offset". If a person knows that they sleep warm or sleep cold, it is still helpful to have an objective and reliable temp rating that they can judge and apply their personal offset to. Certainly WM excels at that.

Maybe you need a Nunatak or Feathered Freinds ...