There are several ways an operating system process manager reports the utilisation of each core, each separately 0-100% or in many cases the sum of all cores. In your case 50% equals 100% of 1 core. Do like the 51% as that would imply 102% :D

ok, thanks for the help on the % matter.
this still unanswered:
is it bad for the computer if the usage fluctuates that much? 0-50 or 50-100 (for 30% setting and 60%set.)
thanks again.

Adding to what Lawrence said, download Speedfan, run BOINC at 100%, and monitor your CPU temperature. If you have sufficient cooling/airflow, it generally shouldn't be a problem.

There's 2 'observations' with the Speedfan utility though it being a superb tool at all times to read out various system/CPU temperatures in the Systray as well as the alerting if things get too hot, even by email:

1. It will on most laptops only show temps but not control the fan(s)
2. On systems that do not have fan(s) allowing variable speed, it's a mere on-off control, which with crunching will most always be On.

Ideally with variable fanspeed hardware, the temperature can be set and a minimum revolution. Experimenting a bit will allow to regulate a semi constant speed with e.g. a desired CPU temperature so no alternations takes place, just a constant 'below irritation level' hum of the fan(s). Of course, keeping these fans squeaky clean (as the rest of the system) will ensure lowest noise, lowest air resistance for optimal performance and lowest CPU temps.