Interpolation of the velocity data is done in data processing between the sample points to increase the accuracy of measured data.
For example, during an 80 km/h deceleration the point at which the vehicle crosses 80 km/h may be between two measured samples, i.e. 80.127 and 79.738. Over such a small period of time as one sample it can be assumed that the deceleration or acceleration is linear, and therefore the measurement in time of where the 80 km/h point was crossed is more accurately measured by interpolating between measured samples. This is used to calculate the exact time the vehicle was at 80 km/h.
Note that no vehicle is dynamic enough to exhibit any behavior that cannot be assumed to be anything but linear between the normal time periods of 20 – 100 Hz VBOX samples.