An on board SCSI controller in reality is a PCI device without a slot. As a mainboard can handle only a limited number of PCI devices, the on board controller shares the IRQ with one of the slots. Newest boards having an AGP slot will share the IRQ for the AGP slot with one of the PCI slots too.
As the drivers of the DIGI32 Series support complete interrupt sharing under Windows 95/98 and NT there is normally no need to assign a different IRQ to the SCSI controller.