The damage previews appear completely bugged in general, not just for this item. I am not sure if they're accurate some of the time, the moment I realized they're not always accurate I stopped looking at them. The tooltips and the portrait display give vastly different values, but I don't think either is correct - the tooltips appear closer to actual damage, but they include all sorts of highly situational damage which skews them for comparative purposes. They also don't appear to list all sources of damage if there's several bonus ones.