99% cpu usage means almost total utilization of single core.
if your system is totally loaded [ couple threads each hogging one cpu ] - you'll see 400% usage on quad core or 800% usage on two quad cores.
processes/threads are re-assigned between cpus - that's why you see 25% of utilisation on each of cores. but you can set affinityset affinity for them.. then they'll stick to selected processors/cores.