If you don't know what your pre-boil and post boil volumes are, you'll never be able to predict the OG. Or for that matter the hop isomerization to predict IBUs. Extracts are really predictable. Steeping grains not so much, but you didn't have any. You either had more volume in post-boil or less extract than you think you had. Or both...20 points off is not likely (maybe even not possible) except from just not having the ingredients and volumes that are entered into the calculator.