Could be either of two things - either the gearbox is faulty or the linkages between the gearstick and gearbox need adjusting - looks like you'll need a mech. head to have a look at it regardless of which one.
i had a vauxhall cavalier for eight years with the same problem although i never got it looked at , i just got the knack of it ! my partner now has a 1995 cavalier which does the same thing , they all have their own little quirks but i would get it looked at if it is causing you difficulty !!