150+ RPM difference in fans due to extension?

I used two 3 pin extensions for connecting Fan 2 & 3 to the motherboard because the cord wasn't long enough and connecting directly to 4 pin molex made them sound like a jet engine.

*i.imgur.com/7wlNkYm.png

Why is there a 150RPM difference between the extended fans and the fan directly connected to the motherboard?

The difference goes upto 500+ when the CPU temp goes higher.

Any solution?

Thanks in advance.