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.
Spikes like that are very very common. Hell, my s9b1 which averages around 13.5THs often spikes up to 20Ths but again only very briefly. Do a refresh and #'s are more reasonable.
As far as I know, it is from the particular data being crunched and not a physical device operation suddenly getting faster. Either for some reason the share diff is dropping as the pool adjusts share rate to the miner or something along that line.
Really need to find the software end of mining Guru's here such as CK or Luke Jr.