Note that you may be carrying an amount of fuel greater than the reserve value you have supplied to the FMC but it may have calculated that the fuel quantity will be lower than that value by the time you are on terra-firma and hence the message.
Just a speculation. I am not too sure about this...
henrik - I saw it happen quite a few times and just ignored it. I have no explanation other than a glitch in the great god 'software'. If your concern is to do with data monitoring I would forget it. Your tech log shut down fuel should silence any excitement.
Then of course, touchdown is not exactly at your destination as far as the FMC destination position is concerned, is it!
- I think only the software coder/FMS spec writer can answer that. Since the FMS has the touchdown lat and long for the selected runway I had assumed it knew where it was. If it is computing fuel remaining at airfield datum...............chocolate teapot?