Ive got three gecko usb sticks that consistently pop up with 18Ghs (set to 12ghs) but they don't reach it regularly, infact they often drop to 8-9Ghs (even though still set to 12Ghs). That's why you have an average hashrate (which is the only one you should be worried about). The next 5 min hashrate could only be 10Ths on your machine so it evens out.