low viral load in HIV positive sample - low viral load in HIV positive sample (Mar/28/2009 )

Hi All
In real PCR , HIV positive sample is in below detection limit.But it is diluted with negative sample 1 in 2 dilution, the result is 104IU/ml. Why does it cause?Negative sample is really negative. Please explain me.
Thanks,
Thin Su

The only thing I can think of is that some times, when you have to much template the reaction is hindered.