When you have multiple vendors/sellers, closing procurements can occur throughout the project during the executing/monitoring and controlling phase as vendor's satisfy contractural requirements and deliver what was promised. Contracts need to be finalized and final invoices paid in a timely manner during this time where you may not have reached project closure yet.
And then once the project's deliverables are completed and the project moves to the closing phase, there still might be open contracts that haven't yet been finalized, invoices still need to be paid, level of quality still needs to be confirmed, etc. So basically closing out procurements may occurs in both phases but it's listed as an activity within 4.7 Close Project or Phase rather than a duplicate process. And also remember 4.7 is an integration process so it can touch upon elements from the other processes.