How is a cnMatrix switch througput calculated

Hi everyone I'm going to show the cnMatrix to one of my costumers, However I have a small doubt, 
How is the cnMatrix throughput calculated?

For example, the Switch EX2028 says it's throughput it's 128 Gbps, but where does this number come from?

The switch is capable of forwarding L2/L3 traffic at line rate. The 128Gbps is the total aggregated traffic throughput when all ports are transmitted and received at their max. port speed. For EX2028 it is calculated as:

24 x 1G + 4 x 10G = 64Gbps

64 Gbps x 2 (full duplex) = 128Gbps