Why non-cluster app outperform cluster app in nodejs?

Many people said cluster module makes node.js app more faster than regular one. But what i experienced here is somehow confusing. I create two scripts, first is the regular http server without using cluster module. Second script is http server using cluster module.

I am using apache benchmark to send high request to these servers. And here are the results:

Attempt#1:
non-cluster: 15,418 req/sec
cluster: 10,333 req/sec

Attempt#2:
non-cluster: 12,563 req/sec
cluster: 9,874 req/sec

how can non-cluster script outperform cluster script?

here is the github repo for the scripts.

via milenko zahovic