load of cassandra cluster are extreme different

Question

I am new guy for Cassandra, i am doing a stress testing in my own Cassandra cluster.
10.191.136.15 is a testing client .
10.191.136.2/10.191.136.3 are nodes of Cassandra cluster .
Cassandra 3.0 .
My table's replica is 1.

I found a problem is that : the load of two nodes(10.191.136.2/10.191.136.3) are extreme different as picture below :

10.191.136.15 sent almost same traffic to 10.191.136.2(141Mb) and 10.191.136.3(138Mb) as picture below

but, why 10.191.136.2 sent so much data to 10.191.136.3 as picture below?
And whether or nor this caused their loads were different.

nodetool status
Datacenter: dc1 =============== Status=Up/Down |/ State=Normal/Leaving/Joining/Moving -- Address Load Tokens Owns (effective) Host ID Rack UN 10.191.136.3 669.51 GiB 256 46.9% 7c80bd3b-9113-414a-a7cb-dfa68d57f300 rack1 UN 10.191.136.2 732.81 GiB 256 53.1% 8b0cd359-ff1d-41d7-b373-e6a6bdb78f98 rack1

Thanks for your help