The deviation here is all based on the headlights, as has been the case with so many cars of late. The XC40's optional headlights earned the highest "Good" rating, which is necessary for the "+" whereas the optional headlights on the XC60 were rated slightly lower at "Acceptable." Otherwise, both vehicles earned "Good" marks across the board in crash tests and the highest rating for automatic emergency braking. On top of that, the forward crash prevention rating was achieved with standard equipment, while many vehicles only manage that feat with optional features.
There's one final nuance to the headlight results we should point out. While the XC40's optional headlights got the highest marks, the standard headlights got the lowest "Poor" rating. The IIHS points out that the reason was for too much glare to oncoming traffic. The XC60's standard headlights, however, were scored a little bit higher at "Marginal."