A pattern is drafted at one base size, then "graded" outward by set increments. If a brand grades the waist faster than the shoulders, sizing up to fix a tight chest can leave the waist too loose — the source of a lot of frustrating between-sizes fits.
Grading quality is a big reason two brands with identical size charts can still fit differently, and why plus and petite ranges often need their own grading rather than a simple scale-up.