Combining two damageable items of the same type in a crafting grid gives the resulting item a 5% bonus in durability, but doing it in an anvil gives them a 12% bonus instead.

How to reproduce:

1. Execute the following command:

   /give @s minecraft:stick[minecraft:max_damage=100,minecraft:damage=90,!minecraft:max_stack_size] 2

2. Enable advanced tooltips with F3+H if not already done.
3. Try to combine the two sticks in a crafting grid and notice the extra 5 durability.
4. Try to combine the two sticks in an anvil and notice the extra 12 durability.

Expected result:

Both ways of combining damageable items would give the resulting item the same bonus durability.

Observed result:

Combining damageable items in an anvil gives the resulting item more bonus durability than in a crafting grid.