In this case, ID.AM-1 maps to 00691 (Establish and maintain a hardware asset inventory.) which is a child of 00689. The citation for ISO 27001 A.8.1.1 is just to establish an inventory for all identified assets. That is a general citation, while the one in the NIST document is more specific. They could both be mapped to the same control, but in this case, when there is a more specific control that a citation can be mapped to, we will map it to the more specific control.
I hope this helps. If not, please respond and we will give more explanation.