Interpreting the medical test result

For the medical test in the lesson (prevalence P(D) = 0.01, sensitivity 0.99, false positives 0.05), we found P(D given positive test) = 1/6, i.e. about 16.7 percent. Explain, in a written answer, why this result seems surprising compared to the test's 99 percent sensitivity, and why it is not contradictory.