Oh, I've got it! Online uses 75% efficiency and Windows uses 70%, by default. Bring up Windows makes them match

The OG calculation between the online "All Grain OG, FG, ABV Calculator" calculator and the Windows standalone program don't seem to agree. With this grain bill: 18.5 kg UK pale 2-row 2.25 kg UK 70L crystal 3.4 kg UK brown 1.7 kg UK chocolate The online calculator gives an OG of 1050...