It may have something to do with how "ab" works, it send more than 1000 requests during the test, and close all connection once number of responses reaches 1000.
It is better to do one test run of
ab -k -n 5000 -c 200 ...
than running ab 5 times with "-n 1000".
In theory, the result should be same, but it is not due to the way "ab" works.