What's the unit for AppRunner's CPU Utilization?

Looking at the CloudWatch metric AppRunner > Instance metrics > CPU Utilization, I see that my instance is reaching up to 1000 of an unknown unit. Well, the question here is simple - what is the unit for CPU Utilization?

Here are my theories so far:

• The official docs page on AppRunner metrics is not very helpful; their description is "The average CPU usage during one-minute periods."
• The most obvious is percentage of CPU, but that should stop at 100%, so 1000% doesn't make much sense.
• According to some other AWS products though, 100% = 1vCPU. This SageMaker question says 400% = 4 vCPUs. So 1000 would be 10 vCPUs.
• But my instances are configured to use 1vCPU & 2 GB. So I'd still expect a max of 100, not 1000. Am I allowed to go above the max for brief amounts of time? Will I be charged extra if so?
• Another theory is that this might be something to do with less than 1vCPU - like AWS Lambda's CPU throttling which lets you use less than 1vCPU. ie, 1000 = 1vCPU.
• Finally, I might be doing something wrong. I'd be happy to share some details of my setup. I'm using maximum for aggregation statistic in Cloud Watch & only 1 instance is running.

But to sum up - does anyone know what the unit is for AppRunner's CPU Utilization?