When you combine two tools by crafting or by anvil, the damage values are different than what they should be. The tools tested were pickaxes, with 1 use left.
Key: <Tool>: Crafting: <Durability of tool when combined by crafting>, Anvil: <Durability of tool when combined with an anvil>
Wood Pickaxe: Crafting: 4/59, Anvil: 9/59
Stone Pickaxe: Crafting: 8/130, Anvil: 17/130
Iron Pickaxe: Crafting: 14/250, Anvil: 32/250
Gold Pickaxe: Crafting: 3/32, Anvil: 5/32
Diamond Pickaxe: Crafting: 80/1561, Anvil: 189/1561

There seem to be patterns. Let's mark the format: Crafting: a/c, Anvil: b/c.
((b-a)/c)= between 0.06 and 0.09. (a/b)=between 0.42 and 0.47, except for the gold pickaxe, which was 0.6.
For the gamemode, I put survival, as that was when I first noticed, but further testing revealed it works in all gamemodes.
Note that the resulting durability is always more than it logically should be.