About memory usage of machine_usage.csv

After anlalyzing the machine_usage.csv, I found that about 50% of machine memory are used neither by instance nor container, for example:
machine_id = 'm_2824' , time_stamp = 461830, all instance used memory about 2, all instance used about 41, and the machine used at that timestamp is 91, the same situation is appeared in machine_id = 'm_1825', time_stamp = 691000. I wonder if the machine system and the management of container and VM costs the last 50% of memory?
Thanks