the reason for this is that the sun does not stand still on the celestial sphere, and the stars do (except for a tiny proper motion). the sun moves about 1° per day eastwards over a great circle known as the ecliptic (which is 360° or a full circle in one year, the annual motion of the sun). as an angle of 1° corresponds to 4 minutes in time (360° = 24 hours), we need therefore four extra minutes of diurnal motion to see the sun back on the same meridian again, making the duration of one rotation just 24 hours exactly (on the average, ignoring small seasonal variations, see equation of time)