Yep, check the conection behind the speedo.
It looks like it's plugged in, but might be a mm or so out.
looks like the vibrations gave you a short "bump" so it was all the way inn again when you were on high revs
must o been a clever feecker theres two almax chains holding it down to the ground have a look at there web site thay cut every chain on the market with a set of bolt cutters except the almax the whole speedo things a bit strange tho the bike was a ex bmw promotion bike i bought it with 600 miles on the clock when i collected it it had 35 miles on the clock now i never checked the speedo on the test drive it was on the trip 1 setting a couple of week later id done 340ish miles went to start it one day and thers just over a 1000 miles on the clock the sales invoice states the milage was 600 plus it had had its 600 miles service any way i kicked up f*ck at the dealer got a new set of clocks and a free service makes me think the dealers can reset the speedos who noes cracking wee bike tho