You should have found that the average speeds are different for different intervals of time. For example, the average speed over the first three seconds is `14.7` meters per second, while in the first second it is only `5.0` meters per second.
You can also estimate average speeds over intervals whose endpoints are not in the table. The way to do that is to first estimate distances at the endpoint times. For example, at `4.5` seconds, we might estimate the distance fallen to be `99` meters — a little closer to `78.0` than to `122.8` because the object falls farther in the second half of the `[4, 5]` time interval than in the first half. That would make the average speed for the first `4.5` seconds `22` meters per second.
The average speed over the last two seconds is `88.1` meters per second, which is
also the slope of the line segment connecting the data points corresponding
to `8` seconds and `10` seconds. When you zoom in on this part of the data graph,
you find the last three points lie almost on a straight line, so the average
speed should closely approximate the actual speed throughout this time interval.