I just figured out why my skirt patterns never fit right

I've been sewing for about 2 years and always had a weird gap at the back waist of my pencil skirts. I thought it was my fabric choice or maybe my machine tension. Last week, I was watching a video from a designer in Portland and she said, 'If you have a sway back, you need to take a horizontal dart here,' and pointed to the exact spot. I tried it on my muslin, taking out a 1-inch wedge, and it fixed the whole thing instantly. I must have made 8 skirts with that same fitting issue. Has anyone else had a specific body shape fix that changed your drafting game?