Tunning nginx/php5-fpm : why low memory usage with 800 users?

I did optimize nginx with what I found on the internet My specs are 2 servers load balanced :

• Intel Xeon E5-1620v2 (4c/8t 3,7 GHz+/3,9 GHz+)
• 32 Go DDR3 ECC 1600MHz
• 3x 160Go SSD Intel DC S3500 SATA3 6Gbps (raid 1)
• 1 Gbps

My optimisation for nginx are:

• worker_processes 10;
• worker_rlimit_nofile 20000;
• keepalive_timeout 15;
• worker_connection 4096;
• The rest of the optimizations can be found here

For php5-fpm (php 5.5.x):

• pm_type: dynamic
• pm_max_children: 300
• pm_start_servers: 5
• pm_min_spare_servers: 5
• pm_max_spare_servers: 35
• pm_max_requests: 500
• pm_process_idle_timeout: 10s

Others:

• Modify the maximum opened file for debian (uopen)
• Modify the maximum connection per socket

The problem is, while on heavy load the 8CPU are 100%, the memory usage is very low (3Go used on 32Go available).

The results of 800 users hitting the application for 1 minutes is available. Caution, keep in mind that there are 2 servers serving responses.

• http://screencast.com/t/vnpRRk12Ry8
• http://screencast.com/t/W126GJPYZH7

Is there any other tuning options for nginx or php-fpm to allow better performances ? The low memory used kind of tells me i'm doing something wrong.