I have one question related to aggregate maximum of graph

I have sum up two ports together but it is actually not accurate

Eg: one port: 10.54 Gb

and another port: 10.95 Gb

So total of max should be 10.54+10.95 = 21.49 but the real on graph is showing 21.34 instead

Can anyone help to explain me about this?

Please check on picture bellow

Appreciate for your explanation

Thanks