Sterak's Gage actually lowered my attack damage when bought?

So I was trying Garen in a solo custom game. I respectively purchased Titanic Hydra, Boots of Swiftness, Dead Man's Plate, Black Cleaver and then Sterak's Gage. Before buying Sterak's Gage my attack damage was 247. After buying it it went down to 237? I'm not too good with game mechanics... but buying an attack damage that ends up lowering your damage is kind of wrong in my opinion? If needed I could attach the runes and masteries I'm using but I don't think there is a link as to why Sterak's behaves this way.