Question about Oracle's interpretation of "host cpu utilization" in v$sysmetric view
The figures in the v$sysmetric view are aggregated from v$osstat over a period of time (either 15 or 60 seconds).

v$osstat figures should closely mirror what you'd see from a tool such as iostat, sar, mpstat, top or nmon, but they will never be identical as it is impossible to synchronise the performance monitoring tool data collection with the data collection performed by the Oracle MMON process. As a result, the aggregated data in v$sysmetric will also never perfectly match monitoring tool output, but should be pretty close.

Busy% is indeed Usr% + Sys%.