Build Advice One Fan Running Faster Than T'Other?

Well I just got my shiny new PSU, plugged everything in, turned the power on, everything is working fine.

But my rear fan is running slower than the top? Any ideas as to why this could be?

Both fans are connected independently of each other to the PSU.

Hardware:

Motherboard
MSI AMD 790FX-GD70
Processor
AMD Athlon II X2 Dual Core 250 3.00GHz (Socket AM3)
Memory Slot
OCZ Platinum 4GB (2x2GB) DDR3 PC3-10666C7 1333MHz Dual Channel
Graphics Card
Asus GeForce GTS 250 512MB GDDR3 PCI-Express Graphics Card
Hard Drive 1
Samsung SpinPoint F1 RAID 500GB SATA-II 16MB Cache
Optical Device
Samsung SH-S223C/BEBE 22x DVD±RW SATA ReWriter (Black)
PSU
600W Be Quiet Straight Power E7, 80

Thanks.

 

are they the same fans?
how old are the fans?
maybe the rear has worn down

 

They're the same.

They're only a bit more than a month old, came with the case.

 

Where are they plugged in and what size are they?
A 120mm back exhaust fan will spin faster than, say, a 140mm top fan, to move a similar amount of air.

If they're plugged in your motherboard, your motherboard could be using fan control to slow the top fan down, as well.