Math Help - graph of problem

1. graph of problem

I'm doing a biomechanics lab report, and have got loads of numbers to crunch. i;ve got to a point where my mathematical knowledge is letting me down.
I've got two data sets: displacement and time, with 5 sets of results for each.
Why when I work out the average displacement over average time, do I get a different results for each displacement over time divided by 5?

If that makes sense

2. is the gradient of the graph changing? i.e. is the graph a curved line? can you write out the data in a table so that I can look at it?

3. I'm not doing graphs atm, just raw data.

I'm using angular displacement/time to work out angular velocity, if thats any help.

I have a mean angular velocity worked out using each (displacement/time)/5, and when I work out average angular velocity/averge time, it works out differently. Which one is theoretically correct?

4. If this is any help:

I'm working out angular velocity, which is change in displacement/time.

Hip Displacement / Time

-9.92 / 0.08 secs
-16.19 / 0.12 secs
-22.57 / 0.08 secs
-13.64 / 0.12 secs
-20.97 / 0.12 secs

So I work out the angular velocity for each one, and then divide by 5 to give a mean, which is -165.89 deg/s-1
But when I work out the average hip displacement/average time, I get -160.17 deg/s-1

Which one is scientifically correct? I am doing all my calculations the long way, so that all raw data is shown, but my group mate is doing the averages, so he only has one final answer for each