Irma,
 
We have had that problem here and there for some time.  No rhyme or reason - even though the person doing the posting swears that no dates were changed, the only reasonable answer we were provided with through various sources was that someone had to have choosen to post the invoice to some date other than the date of shipment.  
 
On rare occassions, I have found that the person releasing product has made a typo (but this has been rare, we have still found invoices posted for a date other than the date the invoices were actually posted for). You may also want to double check the date listed as date shipped, could there have been a typo at time of release?
 
Good luck!