Why does MacBook Pro's fans' speed is different each other I don't think it's a problem, but I thought there would be a reason for it. I'm using a program named Macs Fan Control(gives me to control mac's fan) on my MacBook Pro. 

So there are two fans at MacBook Pro. Fan on the left side's highest RPM is 6156, and right side's highest RPM is 5700. Why these fans' speed is different each other?
Your fan speeds, among other things, are managed by the System Management Controller (SMC). Your MacBook Pro has a number of sensors distributed throughout to measure things such as temperature. 

The SMC uses a range of info (such as temperature readings from sensors) to ensure that your fans operate at the optimal speed to protect your Mac from damage. The fact that one fan is spinning at a higher RPM than another is not a concern. The different fans can operate independently and are each aimed at cooling different aspects of your hardware.