Maybe this is just another 'version' of the bug which is that for creating items, in any case the repair skill of the PC is taken into account, rather than the one of the NPC who is actually using the workbench?
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I encounter this bug as well. My PC has 0+7 repair skill, and <armor name> breaks down to 257 comps. When I equip with more skill enhancements (0+10 repair skill), the same armor can break down to 368 comps. It does not matter who I choose to work on the workbench. So, put all the best enhancements to PC when using the workbench.