Well, that would be because your calculations are completely wrong!
A $2000 computer would likely lose 20% (1800*.2=360) of its value in the first year, so that would be:
2000
-200
-360
_____
=1440
My guess is that you were somehow lowering the discount rate because you've only had the computer for 6 months. You wouldn't do that: it should just be rounded up to the nearest whole year.