constant throughput timer logic

Hi,

I have adequate threads allocated in a thread group and a constant
throughput timer of x. My throughput does not reach x, it only reaches
x-1. Which is correct, the server cannot handle x requests per second.

My question is about JMeter logic. Why doesn't the constant throughput
timer just fire off threads and return server errors? More importantly
how does JMeter know that the application server can only handle x-1 per
second and run at that rate?

Many Thanks

Paul