They’re sized on different fit models.
A typical “misses” pattern is made for a size 6 or 8 “fit model” and then sized up and down from there. A plus-size pattern is made for a plus-size fit model (don’t know the usual number size).
Therefore, size 18 pants are based on a size 6 base pattern. Size 16W pants are based on a plus-sized base pattern. They’re shaped differently, and usually the 16W will be a little roomier.
Sounds like you are stuck in that horrible zone between misses and plus. I’ve been there, and it’s the absolute worst. Misses are too small and plus are too big, and even if you can find your size, nothing seems to fit right. When it comes to shopping, it’s much easier to be comfortably plus-sized.
You won’t find the same garment in both misses and plus sizes hanging side-by-side in a store. They are often listed together in paper catalogs, and usually cross-referenced in online catalogs. Many misses garments are not offered in plus, and vice-versa in some cases.
If you’re in that in-betweeny stage, you’re stuck going back and forth between the misses and plus departments in stores, or between the regular-size stores and the plus-size stores. Catalogs are easier in some senses, but since it’s hard to find something that fits well when you’re in that size range, it’s a pain to keep ordering and returning.
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