uptime is the same as the first line of top(1) or w(1). Load average however (which is what is reported there) is not CPU usage. Load average is the average number of processes *waiting to run/running* over the last x period. The higher the load average the greater the load on the system as the OS queues processes for a time slice. This has a vague relation to CPU usage but that is all. See this for more details.