The tooltip for Spellslinger Headshots shows accurately when there are no purchased items but once you build Calico a little it then shows a decrease in damage.

I also noticed mid-game that inhibitor has the same reaction, though it also shows a damage decrease when you have no items.

I haven't tested on any other heros, just noticed this reaction with Calico

I also noticed mid-game that inhibitor has the same reaction, though it also shows a damage decrease when you have no items.

I haven't tested on any other heros, just noticed this reaction with Calico