Between the high and low figures or between upload and download?
Upload is typically 10% (give or take) of the download speed, for historical reasons.
The difference between 30.02 and 31.78 or between 1.97 and 2.53 is due to the fact that traffic is dynamic - it changes moment by moment. And your speed from point A to point B is determined, in part, by the amount of traffic going from point A to point B.
(Ping is the amount of time it takes for a packet of data to go from you to point B and back - and it has very little to do with speed. But high ping times can kill you if you're gaming.)