It is interesting to note that there are three verses in the Torah that seem to be contradictory.
One Pasuk says בערב תאכלו מצות, the evening of Pesach, we must eat Matza. There is another Pasuk that tells us that we may eat Matza for six days. And there is a third Pasuk that tells us to eat Matza for seven days!
The answer to this contradiction is as follows: The first night is an absolute positive commandment. We are expected to eat Matza at the Seder. The six and seven days are optional. If we want to eat Matza, we may, but we are not obligated.
The difference between six and seven days, depends on the flour we use. If we use flour from winter wheat, we must wait until the second day of Pesach after the Omer begins. Matza from this flour can be eaten for six days. If flour was used that was planted before Pesach, but harvested after Pesach, such Matza can be eaten for seven days.
This is how we explain the difference between one, six, or seven days.