Capacity test on Apache WebServer

I was trying to do a capacity test on an apache web server, but there are some result I can't understand: according to the theory part of capacity planning, I should see three different regions on the plot of throughput in/out.

1. In the first region the expected result is the line y=x, meaning that the web server can follow my requests and reply to all with the code 200-OK (Thus, the throughput I request is equal to the throughput I get).
2. In the second region the expected result is the line y=k, where k is that throughput that indicates the saturation of the web server (Thus, the throughput I get can't go further k).
3. In the third region the expected result is a curve that goes from k to zero, that shows the degradation of web server, which for memory or CPU leaks starts to reject requests.

I have tried to replicate the experiment with a Virtual Machine running an instance of Apache as a server and the Physical Machine running an instance of Apache JMeter as a client. The result that I get is only the first two points, but also if I request a very very huge number of samples/seconds as throughput, I always get the saturation value.

Why I can't get the server going down, even if the CPU is 0% idle and the remaining memory is about 10MB? Or maybe is this the correct behavior and my hypothesis was incorrect? Thank you in advance.