For the other instances you get a specific number of units of processing capacity that you can use 100% of continuously if you like. For the micro instances, you get a base level and build up credits towards bursts, and can not maintain 100% utilization continuously. It's very much different and not the default. To quote Amazon:
> "A CPU Credit provides the performance of a full CPU core for one minute. Traditional Amazon EC2 instance types provide fixed performance, while T2 instances provide a baseline level of CPU performance with the ability to burst above that baseline level. The baseline performance and ability to burst are governed by CPU credits."
A t2.micro allows only 10% of the vCPU baseline performance. Anything above that needs to be "earned" at a rate of 6 credits per hour. The t2.micro can accumulate a maximum of 144 CPU credits (+ the 30 initial credits, that do not renew), each good for 1 minute of 100% use.
So in other words, you can on average only use 100% of the CPU for 6 minutes per hour.
Moving the goalposts here. 'Not owning the whole core' is the default in the cloud.