Faster speed with higher threads

Hi,
My test signs on and signsoff a web site. Ramp is equal to the number of threads.
Here are the results:

(First we recycle all servers)
1 user= 1.402s
10 users=1.707s
50 user=1.087s
100 users=0.813s

Why are 100 users faster on average, than fewer users?

My education in jmeter is expanding but I still have the nagging questions .
Thank you.