The COGS amount will be less than the Jobs Materials Purchased if there are still materials left in the inventory. Please note that COGS will only be realized when the materials are sold. Also, it depends on how you set up your items affecting the COGS account.
When you pull up the report, you can click the amount associated to COGS to see the transactions. From there, you can determine whether these transactions correctly set up. I'd also recommend reaching out your accountant when set up COGS for your items. They can give you better options in doing this and in correcting your reports.
You can also use this article about COGS for more information: Understand inventory assets and cost of goods sold tracking.
Feel free to drop by again if you have additional queries.