I think that might be because there is no single, clear answer. I'll try to summarize my knowledge and the two most related questions:
- Google Earth, Google satellite, and Bing aerial accuracy
- How accurate are measurements in Google Earth?
Let's start by taking Google Earth and even GIS out of the equation and only consider how accurate a measurement on an image can be. If the resolution of an image is a pixel is 1m, and you are zoomed in far enough to see individual pixels (original, not resampled for display purposes), you can measure to within +/- 1m. Now let's get that into GIS, which requires both orthorectification and georeferencing. Both can introduce error. On top of which, you're looking at errors introduced by (re)projection.
Now let's put that all into Google Earth, which is providing you a seamless interface to multiple original sources. All of which may vary in their original resolution and quality of the above processes, so right off there is no single accuracy measurement. This is why Google has that disclaimer about accuracy:
Google makes no claims as to the accuracy of the coordinates in Google
Earth. These are provided for entertainment only and should not be
used for any navigational or other purpose requiring any accuracy
Our imagery varies from sub-meter resolution in major cities to 15
meter resolution for most of the earth's surface, with a global base
resolution of 1KM. Since our database is constantly being updated, we
cannot state a specific resolution for any geographic region.
Google acquires imagery from many different sources with many
different file formats, projections and spectral characteristics. All
imagery sources are fused into a single global database with a
proprietary format that has been developed for the specific purpose of
streaming to our client software.
Source, as linked in question 2 above.
All of that is just for the accuracy of the imagery you're measuring from. Then add in the user interface - how far out are you zoomed when you do your measurements? Far enough in to see those individual original pixels, if they're even available to you? Or out far enough to see the entire field, or the field and its surroundings? What display resolution is your computer running, and how accurate/precise is your hand with the cursor as you mark the boundary? What distortions are introduced by the projection(s) Google Earth uses, which may change depending on where you are looking. The best you can do is find something of known distance near your feature and see what it measures at to estimate the error in that area.
So the short answer is, it varies.