What causes a chlamydia test to be a false negative?

It's just random: Tests miss 5-10% of infections, more by urine, less by urethral or vaginal swab. Maybe other possibilities as well. Chlamydia clears on its own in a few weeks to months, so current testing is negative. Or s/he received an antibiotic for chlamydia (and didn't admit it) or for another infection, curing chlamydia or interfering with the test. Partner(s) of infected person need Rx, even w/ neg tests.