In Shopify, cost is maintained at the variant level and the way COGS (calculated as variant cost x quantity sold) works in Better Reports is that it always takes the latest data from variant costs.
This is different than Shopify's built-in COGS reporting that will not update if cost is entered after a sale.
However, because Shopify does not record historical cost data, there is no way for us to measure how much the cost of a product was when it was sold.
In Better Reports, as a conservative measure, variants without a cost will incur 100% cost of goods sold and 0% gross margin (Cost of goods sold = Net sales).
However, you can record your variant costs in Shopify to rectify this. You can see the link at the top when you visit the built-in report 'All times sales' or any other report with COGS data.
You can refer to the built-in 'Variants without cost' report that shows a list of variants without cost recorded. The report will update and show the current cost once you have updated the costs in Shopify.
If you have a preferred calculation of COGS for variants without cost recorded, feel free to reach out to us at firstname.lastname@example.org! We'd be happy to help you create custom COGS fields with your preferred calculation.