Order of Notifications When Multiple Approvers Exist

In Payables, we have a BPM rule that sends notifications to the requester. Sometimes, different requesters are defined on different invoice lines, and they all receive their notifications in parallel. However, we do not understand the order in which these notifications are sent. It is not always in the same order as the invoice lines, nor in alphabetical order of the requesters. Does anyone know what determines the order in which the notifications are sent?