Note that is ground speeds. Pilots generally fly to an indicated airspeed or mach number.

Being a smaller plane it has less inertia so will accelerate and decelerate faster, than say a wide body. Also what was the wind like? The smaller the plane generally the more it will bounce around more in the wind.

The data source is ADS-B so the speeds and altitudes are coming from the avionics. Not math at the receiver end. If the message is received the data is accurate.

Also it is a dh8c not dh3. That plane is Dash 8-300 not an DHC-3 Otter.

Though also a bit unclear what your question is and your hypothesis for the presentation.