I've had an experience with one bowyer where the listed DW@DL was off by 2 to 3 lbs. from what was listed on the bow. That said, it wasn't a big deal to me. When I bought it, the seller put it on the scale to inform me the weight was more than listed. I know with Black Widow, you can pay a little more to hit the weight you're wanting (+or-1 lb), if memory serves.
That said, if I see a bow for sale that's listed as xx@31" DL, I will pass up that bow because I believe that bow was designed to hit optimum performance at 31" If I draw 27.5" it would not be to my advantage to purchase a bow to hit a certain weight at 31"