“from your harvest.” This is a very accurate statement because the grain was not harvested until it was ripe, and during the act of cutting the grain and carrying it to the threshing floor some of the grain would be knocked off the stalk and fall to the ground, where it would grow. That “volunteer grain” was not to be harvested in the Sabbath year.
Although this seems somewhat harsh, it taught the people to rely on God and not be sneaky about it. If people could eat the volunteer grain on the Sabbath year, then they would almost certainly give in to the temptation to “allow” more than just accidental grain to fall to the ground the year before the Sabbath year. In fact, quite a bit of grain would likely be “accidentally knocked off the stalk,” by people as they harvested, in full knowledge that it would grow and be food during the Sabbath year. God wanted to avoid such dishonesty and teach the people to rely on Him.