Wow. 3qt/hour boil off is nice. If only I could get that low. (I'm in the 1.7gal/hr range) And with such insanely low grain absorption, you must be squeezing the everliving snot out of that bag. I do what I can, and the resulting spent grain is pretty crumbly, but never that dry.
As long as your volume measures are really really accurate, yeah, that can be a problem. I'm still going to hazard a guess that 'resolution of measurement' is at play somewhere. But more likely, just back off the heat on the decoction a bit as that loss seems really high percentage wise. If you can get that down, of course, you're infusing more volume.
Do you go straight to boil with your decoctions? Traditional method is to step through the remaining rests with each decoction, *then* boil. Which of course adds more time, and more opportunity for lost heat in the main mash. (requiring some sort of external insulation like bubble foil wrap, blankets, etc.)
What are you using for a mash tun? Dimensions?
And thanks for the specs. Have you had a look at
http://braukaiser.com/wiki/index.php/Braukaiser.com and his spreadsheets? I've read over his decoction pages and that's pretty much what we follow in most cases. For any brew, his sheets also take into account sugar volume from grain. I don't think I've seen anyone else do so.
I'm about to start playing with his Party Gyle spreadsheet.