You might be able to adjust the sampling rate of the GPS. It basically works out your position, waits a while, then works it out again, and so on. From those position readings, it calculates heading, speed, average speed and so on. The more frequently the GPS samples, the more accurate it is, but the log files it stores will get much bigger, and the GPS may even run out of memory unless it can store the data on a huge memory card. (The memory on my old GPS is strictly limited so I have to be careful not to overflow it.)
The GPS effectively considers your ride to consist of lots of straight-line segments joined together.
It might be that you were heading towards the field, then the road curved round before the GPS sampled your position again and found that you were not in the field after all!