The logical assumption is that the chop or turbulence is caused by vws with up and down drafts, changes in SAT, or changes in the direction of the wind. Any updraft will cause the aircraft to climb but as the autopilot is maintaining altitude it lowers the nose to maintain it and since the autothrottle is in a lower activity cruise mode it allows the speed to increase. Likewise if the wind shifts,- increases on the tail, or reduces on the nose,more often than not there's a speed increase. Since this is not as noticeable on the 744 it most probably is due to the wing design being so 'slippy' hense be very careful when close to the barber pole, the auto throttle doesnt always get more aggressive in it's response. However there are occasions when the speed does drop first before entering turbulent conditions, which can be also noted by a change in SAT.