Why Do Pregnancy Tests Have Different Results?

my daughter found out 7 days ago she was pregnant, after being 8 days late for her period .... she had 2 pregnancy tests at the drs both positive, also took a clearblue digital, also said pregnant, the dr sent a urine sample to the hospital which came back negative, but the same day another clearblue said pregnant ... she started to have a very light bleed over the weekend only requiring 1 pad a day and it didnt get full (sorry tmi) lighter than her normal period, all the drs have said it seems normal ... as she is not bleeding heavy, no pain etc .... so why has a pregnancy test now come back saying negative ??? she is so confused