Why all the bumps?
FWIW, I my first brew was a 5 gallon batch with 6.6# of amber LME and a pound of crystal, and the OG for that turned out at 35, compared to the predicted OG of about 50. So I had that same issue, and I don't really know the answer. My prevailing theory is that we're not mixing the wort up well enough after we top up to 5 gal, so more of the sugar is sitting on the bottom of the container. Since we typically draw our gravities from the top, we get an artificially low reading. I could be completely wrong, but it's the best I can come up with