aCPU stands for Average CPU. In shared hosting majority of customers have their sites hit very infrequently, often less then once a minute. Once you average CPU usage for such a customer across 1 day (or an hour or even 10 minutes), the actual CPU usage will often bellow 1%
This is why when you run lveinfo, you often see aCPU to be 0 or 1. If you see some user using 5-6% of aCPU, this user is using a lot of resources.
You can probably have no more then 10-15 such users on a server (15 users *5%=75% of total CPU resources of the server). So, such users should be either upgraded to VPS, or charged correspondingly to their usage.