Queries with same explain plan takes long time on Node "b" in RAC db env compared to Node "a"

We have 2 node RAC db env. for our Enterprise Datawarehouse env ( cidcslpedw01a & cidcslpedw01b are the servers name ). But we have observed that same query with same execution plan runs in less time on node a ( cidcslpedw01a ) while it takes time on node b ( cidcslpedw01b ). As per our primary analysis, all the initialization parameter, hardware configuration are same on both the nodes. Moreover execution plan of the queries are also same then why node b takes long time to execute the same query comapared to node a ?

Thanks.