Some orders are showing fullfillment details for the given orderId but order API is giving not found

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So basically this API `/admin/api/2020-04/orders/#{order_id}/fulfillments.json` fetched fullfillment result.
Whereas when calling `/admin/api/2020-04/orders/#{order_id}.json` with same OrderId gave "not found".
How is this possible.
I checked with multiple orders it gave similar error.
Please help me know if this is an error or normal thing.
Thanks in advance.