How many PO & AP Periods are open at your end?
You mentioned : 1) User obeserved that some line of a PO is generated in Nov period but this lines are not generated in Oct-2009..
Is the PO is a BPA? If the PO lines generated for a Open Po, then creation date will appear.. What's the concern here?
2) You again mentioned : i observed that Invoice matched this PO is put on hold at Sep-2009 and released on Dec-2009. i dont understood this issue.
Check who has applied the Hold (any user or it's a system Hold) and released by whom...