The Yismach Moshe explains that the number of shabbosim in seven years comes out exactlly as a solar year. There are 52 shabbosim in a year plus one extra day 52×7 =364 plus 1 = 365 days
7 yrs x 52 =364 plus the one of 7 left over = 365 days. This shmita year was a makeup for the improper observation of shabbos in seven years to rest a year every seven years.