Giving Summary fields were only recently added to Account Records. The "Combined" fields on the Contact records include calculations of both hard and soft credits. Someone cannot give on behalf of an account, so there are no soft credits associated with accounts. Because of this, the "Combined" fields (as well as the Soft Credit fields) are not intended for use. 

The reason those fields exist on the Account Record is that they were copied from the ones that are on the Contact record. They are there for when Soft Credits are eventually implemented at the Account level, but there is currently no ETA for that functionality at this time.