Why node.js+mongodb does not gives 100 req/sec throughput for 100 req sent in a second?

I kept node.js sever on one machine and mongodb sever on another machine. requests were mixture of 70% read and 30% write. It is observed that at 100 request in a second throughput is 60req/sec and at 200 requests second throughput is 130 req/sec. cpu and memory usage is same in both the cases. If application can server 130 req/sec then why it has not server 100req/sec in first case since cpu and memory utilization is same. machines are using ubuntu server 14.04

Answer

Make user threads in Jmeter and use loop forever for 300 sec. Then get the values.