When the store "detaches" the SKU from a product to be exhibited individually on the storefront the ProductImpression from the GTM only sees the products by its productID, not by each SKUId. So if the product has 3 different SKUs on the search page, the ProductImpression will only show it as one product.

Go to any store that "detaches" the SKU from a product into individual products and try to see the ProductImpression from the Data Layer on GTM.

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