The boil.
Interestingly the pre boil gravity were both exactly the same. Final gravity the BM which lagged behind the boil of the Guten came out at 1,051 and the Guten 1,048. theoretically should both have been the same
My last few brews have been full volume mash in the Guten and my efficiency has suffered a lot, my last brew was supposed to be a 7.2% IPA that's come in at 4.9%, a nice beer but I'm now calling it a 'session IPA' that was from a 6.2kg grain bill so I should have got more than that. I'm going to go back to sparging with ~6kg mash and ~30L of water the recirc pipe ends up in the grain bed and its just not extracting enough sugars, with lower volume of water I can hopefully get the top plate on and have the water flow better over the grain. My first few brews with the Guten when I did sparging I ended up the other way with too high gravity but I suspect that was boil off rate I had wrong in BS.
Never a dull moment in this game, always something new to try.