This, I suppose, is my question. For this batch, I used an all grain recipe kit from northern brewer (see link below), and followed the recipe exactly. I don't usually use kits, but because I was trying something unusual here, I wanted the base nut brown ale to be a known factor.
The only thing that I did differently was to use fresh maple sap instead of water (for both the mash and the boil). I pre-boiled the sap ahead of time to make it suitable for mashing, so I do not think that anything unusual got into the beer that way.
http://www.northernbrewer.com/nut-brown-ale-all-grain-kit
I understand the math, and that is not what is confusing to me. The thing I don't get is why a wort with extra sugar in it (maple sap is about 2% sugar, 98% water) would have produced a lower alcohol beer. The nut brown ale by itself should have given me a beer in the 5-6% range, give or take.