When combining tools in a crafting table/grindstone or anvil, the resulting durability of the new tool is more than it should be

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.