It shouldn't come as any surprise that, like other web host providers, A2 Hosting's unlimited plans aren't actually unlimited. It expect you to use its service like "similarly situated customers." This is like being on a highway. If everyone is going a few miles above the speed limit, you're probably okay, but if you're barreling down the fast lane past everyone else, you're probably going to be asked to slow down.
I was running a small private weather website in AWS and the satellite images got "picked up" by a news website and they regularly use them during major weather evenings. AWS' 12c per GB of outbound network traffic made things expensive and VPSServer makes this a lot more manageable and has excellent data volumes included with the price of the VPS. I also get many more CPUs for the price compared to AWS, so I am a happy customer.
A Content Distribution Network (CDN) is a network of servers distributed geographically. The purpose of a CDN is to deliver content on the internet much faster. If a user in Toronto is trying to access your website, the closest server to them will take care of this. But someone coming to your website from Sydney, Australia, will be catered to by a server closest to them. The main host server holds your website content, and all the other servers have a cached version each.