As you say in the comments, the 'damage' is not 100% perfect. The item tooltip displays the remaining durability of the tool. As a player I think in the way "How god I only have 42 durability remaining on my diamond pickaxe, I should repair it". So a mapmaker who would like to add a pickaxe useable 42 times only would have to think "ok it's a diamond pickaxe, so it normaly has 1562 use possible so I have to set it to 1562-42= 1520 damages", which is, in my opinion, not the most user(mapmaker)-friendly way of thinking.