When you combine two tools in the inventory crafting menu with their durability adding up to less than the total, for example one stone pickaxe with 29/131 and another pickaxe with 63/131 durability, the resulting tool will have an inaccurate durability number which does not equal to the sum of the two tools that it was made with.

Steps to reproduce:

1) open a survival world in 1.18.1

2) make two stone pickaxes and lower their durability until their combined durability does not add up to the total durability of that tool

3) open the inventory ui and combine both pickaxes in the 2x2 crafting table.

Expected result: the durability would add up to the sum of the previous two pickaxe's durability

Actual result: the durability is slightly higher or lower than the sum of the previous two pickaxe's durability

Please note that there were some mods being used at the time:

Client side: Optifine (is not known to affect game behaviour)
Server side: Fabric-api, Fabric-discord-link, lithium-fabric-mc (none of these mods are known to change game behaviour)