Hi Friend,
I have checked same on VC 5.1 & host added to the cluster was ESXi 4.0 : For me its as expected in both places
%memory was 6 & from host it was 1035 MB/16379.31 MB.
I also checked on VC 5.5 & host added to the cluster was ESXi 5.5: Here as well its correct.
%memory was 16 & from host, it was 1055 MB /8182MB .
From my verification, its simple arithmetic calculation for calculating % memory.
Can you please let me know which vCenter server version you are using? I will check on ESX 5.0 U3 + latest patch & VC 5.0U3 after some time.