Related Interleave controller (was: Question about users.xml sequence)

This is a (loosely) related question on Interleave controller and sequence of http requests My test plan has a Interleave Controller Http req 1 Http req 2 Http req 3 Http req 4 If I have 1 thread, the requests are rotated round-robin. If I switch to, say, 20 threads, then I will see

Question about users.xml sequence

I am using HTTP User Parameter Modifier and users.xml to set up more than 100 users logging in a system. It works fine, however, JMeter seems to pick userId/password randomly. That is, it won't use the first userId/password for the first thread, the second userId/password for the second thread, etc