People aren't computers. They're like human that way.
False negative readings can occur when testing is done too early. Quantitative blood tests and the most sensitive urine tests usually detect hCG shortly after implantation, which can occur anywhere from 6 to 12 days after ovulation. Beta hCG levels rise exponentially in the first two months or so of pregnancy so the earlier the test is performed, the higher the chance of a false negative result. Less sensitive urine tests and qualitative blood tests may not detect pregnancy until three or four days after implantation. Menstruation occurs on average 14 days after ovulation, so the likelihood of a false negative is low once a menstrual period is late.
False positive results can result from diseases like choriocarcinomas, IgA deficiencies, heterophile antibodies, enterocystoplasties, gestational trophoblastic diseases (GTD), gestational trophoblastic neoplasms (GTN), and testicular germ cell malignancies. Also, many home pregnancy tests show a positive or unclear result when read well after the suggested 3-5 minute window, independent of an actual pregnancy; this type of false positive is also known as an evaporation line. Additionally anyone who has been given an hCG injection would also give a false positive.
Some individuals react to some substrate in the test and thus will display a consistently low positive blood pregnancy test even though they are not pregnant. This phantom hCG may lead to serious misdiagnosis and intervention, but can be detected with serial dilutions. Patients with phantom hCG have a positive blood hCG but a negative urine hCG test.