I am not sure, if this is a feature or a grid error.
This image is from Town Farm Plot in Pumpkinvale, perfectly aligned and allows the entire area to work. In fact this terrain is symmetric in its entirety. Its size inside the fence is 70x56. The same goes for Beach Farm Plot in Wahoo Beach.
Below images of River Farm Plot in Pumpkinvale, areas that do not allow work, is asymmetric and its size varies according to the side.
Fence in the air Squares unavailable
The same goes for Field Farm Plot in Pumpkinvale:
Also at Jungle Farm Plot in Wahoo Beach:
This other case, Hill Farm Plot in Wahoo Beach, if it is symmetrical but its size is 70x55, because of the position of the fence of the entrance and the back you lose the additional row to complete the 70x56.
The worst case is the Spoonie Farm Plot, it is totally asymmetric and in fact I did not manage to count its size.
I leave them for your consideration.
Note: I loved that extra row outside the fence, it's perfect for creative flower decoration!
I was also reviewing the wallpaper, a pretty solid function, it was a pleasant surprise!