An interesting fact that I found out is that my back is shorter than the pattern by one inch, but it is not short from the armhole to the waist, but rather between the bottom of the armhole and the shoulder. I also discovered that my arm is shorter in the upper part above the elbow than the pattern, but longer between the elbow and the wrist. No wonder short sleeve blouses always end up below my elbow. Making up one of these fitting patterns may reveal some interesting facts to you about your body. One thing for sure, you will learn something about why you can't just buy a pattern and make it up and have it fit.
Few people have the same proportions as the models used by the pattern companies.
The benefit of making up a muslin from this sloper pattern is that you will learn which alterations you need to make to all patterns. If you have erect shoulders, they will remain that way through every pattern and you can learn to correct this problem before sewing up a garment and then trying to fix it by shoulder pads, etc. If you have a curved upper back, you need to learn to adjust the pattern for this, so that all patterns will fit you better in this area. This process of making a muslin is not guaranteed to solve your fitting problems forever, but it will enlighten you about your own body and you will begin to learn how to fit yourself. Go slowly through the process.