Hi Mapleloor
Unfortunately MYOB's inventory module is a little like that - unreliable - you will see posts all over the forum with different un-explainable examples. Sometimes it picks up figures randomly and then creates astronomically high COGS amts or on hand Values (into the millions!).
From personal experience I would say the bulk of the errors are caused by editing or deleting invoices/bills. MYOB incorrectly updates things in the back end.
It is essentially up to the user to manage and control inventory.
The only solution I can suggest is to record an Inventory Adjustment Journal to set the SOH and Value to what it should be today.
Regards
Gavin