Measurements

So I found a dress on ebay that I absolutely adore (in gold/burgundy), except that it’s a corset back. But I was like, well, maybe I’ll try it and a seamstress can fix the ass shelf issue. The place accepts returns, so why not, right? Well, first, I need my measurements to compare to the size chart. Scott & I took a pilgramage to Jo-Ann Fabrics to buy a tape measure of the sort for measuring people (not to be confused with the kind for measuring other stuff like walls and lumber). We took my measurements:
Bust – 44″ at fullest point
Waist – 40″
Hips – 54″ at fullest point
Well, that puts me in the 22-24 sizing on this dress, which was not the one in stock, sadly. 🙁 But then I started poking around. I buy clothes from the Avenue all the time. I wear a size 18 there (16 if it’s spandex enhanced or stretchy). Here are the measurements for a size 18: 45-36.5-47. Now, per their size charts, I should be wearing a size 26 in their clothes…but I don’t. A size 26 would swallow me…heck, stuff in 20 or 22 falls off me these days.

I pose the question to the knowledgeable out there…what’s the deal? Am I possibly measuring wrong? Am I supposed to really tighten the tape measure on me? Are size charts generally vastly incorrect like that?