AMP doesn't typically cross check against the Posts from different sources as our Conditions of Sale should have an upfront agreement as to whose Posts will be used to finalize the purchase.  

That said, there are some baseline things you can check to see if you're even looking at an even comparison (which usually is not the case & why the Posts are different):

• Do both Posts show same number of spots & dollars?  
• Are both Posts using the same Monthly Survey Book?
• Are both Posts using same period of days in analysis?  
• Are both Posts pulling same assessment of measurement for airings that cross midnight?  

Typically you'll find that one or more of the above is not comparable from one report to another and that's the culprit to those differences.  
If you have a yes to all of those, please put in a ticket with Platform Support and we can take a look.