Not All Sleeves Are Created Equally

Age is just a number- we have all heard that before. Well, the same applies to sizes, or does it! Small, Medium, Large..... what size do you wear? Well, I'm a 2 or 4 in one brand, a 6 in another brand, and in that European maker brand, I am an 8. That is quite the run of the gamut for sizing, While rifling through my closet, I found that I have a variety of sizes, and guess what, they all fit me in one way or another (or why would I buy them). However, you would think that a size Small in one brand fits (hooray)- thus I buy all of the Smalls in every color of the same item. Only to get them home, and surprise, they do not fit the same way. How and why is this possible? Is this quality control? I tend to find the lower the quality, the higher the variance of size difference vs a more consistent size with the higher the price of the product. This seems to make sense as a higher quality garment will have most likely more care, time and checks and balances in the design and production process. Which made me wonder- is it better to sacrifice style or quality for a size?