There are various reasons why multiple POs can be raised from a single PR.

Firstly it is important to note that there is a separation of functions between the Warehouse Organization that raises PRs and the Purchasing Organization that created PRs.

Warehousing may require multiple units of the same item and will raise single PR. Purchasing may however need to split this PR into multiple POs, due to various reasons including availability and pricing differences from various suppliers which the Warehouse organization is not aware of.

Warehousing may raise a PR with multiple different Items on it. Purchasing may need to split this PR into multiple POs as the different items need to be procured from different suppliers.