Hosting that is aimed at specific CMS platforms and tweaked optimally for them: e.g., WordPress hosting, Drupal hosting, Magento hosting, etc. In these cases, the best hosts do far more than just provide the minimal requirements for those platforms. They provide a rich, and fully managed environment so your site can perform like a well-tuned machine.
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.