OK, than again my older question: the dialog allows to select _BoincTasks and all known machines. The _BT's graph displays BoincTask's CPU usage (err, does it?) What do the single machines' graphs display?
Expression "the BOINC worker threads / computer" might be understood as "number of ???? threads per computer", but that does also not bring me further 
_BoincTasks is the process run time %, so everything combined.
The others are the internal threads (BOINC only) that handle everything for that computer, like history, tasks request.
All samples are 10 seconds long. So when BT consumes 0.1 Second of a core, in that 10 second interval, it shows 1%. That's the spike, the thinner line is the average (10 minutes), more understandable by humans.