Statistics of DynamoDB with CloudWatch

I get ConsumedReadCapacityUnits statistics graph of DynamoDB table, the Sum statistics was like this:

               +-------------+
               |             |
               |             |
               |             |
               |             |
---------------+             +-------------------

but the Average statistics was like this:

---------------+             +-------------------
               |             |
               |             |
               |             |
               +-------------+

How to explain this statistics?