Apache httpd 2.2 starts with ThreadsPerChild exceeding ThreadLimit warning message

Issue

I got the following warning log message when I start Apache httpd:

WARNING: ThreadsPerChild of 100 exceeds ThreadLimit value of 64 threads, lowering ThreadsPerChild to 64. To increase, please see the  ThreadLimit directive.

Even if I configure ThreadLimit and ThreadsPerChild correctly (setting same or setting ThreadLimit larger thant ThreadsPerChild):

<IfModule worker.c>
StartServers         4
MaxClients         400
 :
ThreadLimit         100
ThreadsPerChild     100
MaxRequestsPerChild  0
</IfModule>