The company I used to work for migrated from Wang VS to AS/400. On the Wang, if you were the only user, you got the full load. I noticed on the AS/400 that even if you were the only user, you never got the full capacity of the machine.

My guess is it works like that to make performance more "consistant" for the average user. On the Wang we'd get calls in the afternoon about slow performance, but never on the AS/400 - the performance drop wasn't as noticeable.