You're asking about the false positive rates of syphilis blood tests in primary syphilis. It's a biostatistics thing.
The AAFP has a good article on the "clinical dilemmas of syphilis."
The VDRL and RPR, respectively, are reactive in 78 percent and 86 percent of patients with primary syphilis. They become positive within approximately four to six weeks after infection or one to three weeks after the appearance of the primary lesion. Thus, these tests can be negative in early syphilis, when patients have lesions.
This means that a test could be incorrectly negative if:
The person is not in the 78% (VDRL) or 86% (RPR) of people with the disease that the test is able to identify correctly
The test is done before the body has made enough antibodies for the test to detect them yet
FTA-ABS is often used as a confirmatory test after a positive VDRL or RPR. Sensitivity is 84% in primary syphilis and nearly 100% in later stages. Its specificity is 96%.
Usually clinicians start with a screening test like RPR, then if it's positive, follow up with a confirmatory test.
Ideally screening tests should be highly sensitive (rule out); confirmatory tests should be highly specific (rule in).
With something like syphilis, often a chancre is treated as syphilis even with a negative test in case it's a false negative, due to the serious transmission and complications possible from untreated syphilis.