Original Gravity explanation??

I just finish a double brew day this past Sunday. They were my 4th and 5th batches since I started home brewing in August. My first 2 batches the Original Gravity was off and I figured out why. My 3rd batch I nailed it as with the 4th batch. I was feeling pretty good until my latest batch, the 5th batch. It is a double IPA or Imperial IPA. The Original Gravity should of been at 1.072 (according to the recipe), but mine was 1.042. I took the reading after oxygenating it (just like the previous 2 that had the correct O.G.) One thing that I did different was used a combination of Liquid Malt Extract and Dry Malt Extract. The recipe called for .5lb Munich Liquid Malt Extract (I used all liquid for this) and 8.5lbs of Pale Liquid Malt Extract (for this I used 6.0lb of liquid malt extract, and 2 lbs of light dry malt extract. I used 2 lbs instead of 2.5 lbs dry due to subtracting 20% of the weight when converting from liquid to dry).
The only other thing was my hops added at 0 min (1.5oz) I forgot until about 5 minutes after taking it off the heat. What are your guys thoughts on all this? I still think the beer will turn out great and it is fermenting like crazy as we speak. Thank you in advance.

I have had this happen before in my first few brews - I have a couple under my belt my still consider myself to be a few shades over novice. One thing I hypothesized was that after I added water that it wasn't property stirred and mixed so the readings could be in accurate - much like mixing sugar in the tea and coffee - anyhow I would be interested to see what the more experience brewers here would say but here is a former thread that sheds some light perhaps that can help you - https://www.homebrewtalk.com/f37/missed-my-original-gravity-quite-bit-313222/

Your gravites weren't off, your gravity READINGS were.....that's not the same thing.

Read this.

Attention new brewers, yes your original gravity reading is wrong. Don't panic.

That was a great post! Thank you. It seems like the final gravity is a little more important so to know when the fermentation is complete. Would this be a true statement? I'm thinking also that the OG may not even be worth taking with the type of brewing I'm doing.