Why is the amount charged on my invoice different from the amount on my service agreement?

The reason is that the service total on the service agreement does not include taxes and fees.  

See the below section from our service agreement: 

This is the Service Order Billing section of the service agreement. It lists both monthly recurring charges as well as one-time charges and it explains that these totals do not include applicable sales tax or required telecom regulatory taxes and fees. These are calculated each time an invoice is generated based on your location.