Hi Brad
SAP application help contain explanation of keyfigures which is quiet good. I've put the link to the same at end of this response.
You mentioned contract spend to be greater than total spend. It would help if we konw exact fields you're talking about here. A possible case where contract amount is higher than spend would be when total amount of contracts for given period(contract amount) is greater than value of total invoices posted (spend).
Regards
Rishu