Your Total CPU Resources of 383 GHz is the sum of the CPU resources from each host in your cluster. In your example I am assuming this is a 3 node cluster, with each host having 48 cores at 2.66Ghz ... 3 x 48 x 2.66 = 383.04.
If you select a single host, and the configuration tab, there should be a "System Resource Allocation" menu item listed in the Software block ... with this selected, there should be an advanced setting on the right hand side where you can see reservations for all the internal system resource pools ... my assumption is that if you add up the reservation of all of these (multiplied by the number of hosts in your cluster) and subtract them from the total resources you will end up with a "guaranteed resource allocation" which is seen on the resource allocation tab.
I'm not sure if you HA admission control settings influence this as well, but this should be a good start to understanding the difference.
Cheers,
Jon
vExpert 2014 - 2022 | VCP6-DCV | http://www.jonmunday.net | @JonMunday77