This example simply isn't what you would call perfect English. Perfectly understandable, but the author was probably more worried about the technical meaning than of the grammar. Any of the following would be considered 'more correct':
If the number differs, the PCI bus orders that the packet be re-sent.
If the number differs, the PCI bus orders the packet to be re-sent.
If the number differs, the PCI bus orders a re-sending of the packet.
Remember also, that for technical manuals, the author's first language may not be English, and that they may be more used to speaking than writing - you can be a lot more informal with spoken grammar without anyone picking you up on it.