I went small when switching to AG, and built a convertible mash tun (meaning i could take it apart and put it on a larger tun) at 2 gallons. My biggest issue was my Brewhouse efficiency. mine was at 70%, and I was still way lower than my targets. yours at 75% is probably too high as well.
Another thing, initially when i was trying to figure it out, I was taking gravity readings with hot wort, and not calculating for the temp changes. Once I calculated for temp differences (I use this calculator), I was closer, but still off. A combination of these may be partially to blame.
Again, these are from my experience (with 1 highly irregular setup), so none of this may apply to you.