It sounds as if you are trying to reconcile usng the Online Statement feature rather than looking at the Billing Statements. If a transaction occurred in December but was delayed in processing it may display online with its original transaction date -or- with the posting date. If the same transaction has not appeared on more than one Billing Statement (which you can see by looking at the PDF version) you have not been overcharged.
If a charge is listed by the posting date rather than the original transaction date there should be an asterisk next to the date in the Online Statement.
I'm not saying this isn't a problem, but it's not an overcharge.