Does the customer have stock, settings, stock settings - "Chronology in Stock" turned on "Per Location"?

Also, system, setting, locking has a checkbox to prevent users from future dating any stock updating records that must be ticked - because if you future date anything stock updating, posting anything to today's date is back dating :)

If it is "None" then users can backdate stock transactions and likely they have backdated new stock updating transactions (or OKed any not OKed stock updating transactions) after running stock list report to excel.

I have all my customers with Chronology checking set Per Location, future postings are blocked, and their stock list matches with NL every da (except productions with quantities more than 3 decimals still cause some small rounding differences at times which I haven't been able to replicate and fix :))