Question:

Why doesn't the disassembly and the assembly cost match?

Answer: 

If the costs of the assembly and the disassembly doesn't match this is due to the differences between the calculation of disassembly and assembly costs.

Assembly costs are calculated using the Average Landed Cost(ALC) of the component products.

However, disassemblies are calculated using the ALC of the assembled product. To fix the cost you may need to conduct a stock adjustment to adjust the costs of the products.