Wouldn't it be logical to assume those were in fact cartwheel tracks? Seeing as they were using those to transport Maximus away from his house, one could assume that there had been a cart moved over the land at other times too...
The marks you can see in the fields are what we farmers call "tramlines" and are most certainly a product of modern machinery so this is a genuine slip up. Tramlines are created by clever technology used on todays modern drills. As most farmers today spray and fertilise their crops, to avoid over application of product, they need a visual path to follow to make sure they are not overlapping and puting to much expensive product on the crops. Thus depending on the width of the implements they use (typically todays sprayers are well over 15m wide) the distance apart the tramlines need to be is programmed in to the drill and at the right point on a particular run up or down the field the drill will deliberately not allow crop seeds to make it to the ground so obviously no crop can grow here as it was never planted!! The gap that is left is what you see and is the route that tractors and sprayers will follow in the future. I dont know of any Roman who had a john deere tractor let alone a combination drill!!!
How can you see the width of the actual tracks in the ground? You can only see the paths in the crops caused by tracks of an indeterminable width. True, they almost certainly are tractor tracks but then it is possible to see them as cart tracks in line with the date of the story.
Have you ever gone on the highway after it rains? You would notice that the dry spots on the road are much larger than the width of the tires. This happens because no-one drives on the same spot. The same could happen with the cart tracks. :)