If multiple SKUs share the same FNSKU, Amazon sees them as one product with one shared inventory pool. For example, if you have 100 units of that FNSKU, and it's used across 3 SKUs, those SKUs are all pulling from the same 100 units — not 100 each. Showing each SKU separately would make it look like you have 300 units, which would lead to inaccurate restock suggestions.
If a SKU has its own unique FNSKU, it will appear as a separate line on the report.