You wanted to know what is causing the creation of many Purchase orders when bundling Purchase request Items of same Purchase request.
Purchase order is not created as per Bundling rule you have chosen.
SAP Business ByDesign
Along with bundle and order option that you choose system also considers the currency of each Purchase request item to split as per currency and create Purchase orders separately for each currency.
This is generic split rule existing in system currently.
2706196 - How the Currency of Purchase Order that is Created From Purchase Request is Determined
Wrong Purchase order, Purchase order created wrongly , KBA , AP-POP-PO , Purchase Order , Problem