I just finished a Black IPA (OG 1.075. I brewed it using a 6 gallon full boil. I measured the OG as 1.066. I thought i did everything right. I did leave a bit of trub in the bottom of the kettle. What causes the OG to come in low? It was a kit and they suggested using a 2.5gal partial boil. Can this make a difference?