Just like an airline might sell 110 seats on a plane that only has 100, hoping that 10 people will cancel or not make their flight, so too do many hosting providers oversell their resources
hoping that every website won’t use those same resources to their full allowance.
Most often this is a calculated risk with a low chance of occurring, but if companies get greedy and oversell too aggressively, that risk increases drastically. Worse still, if your server database goes down, not only will your site be impacted by downtime which can cost you a lot of money, but it can also cause database corruption, which might make certain elements unrecoverable.
To avoid this risk, as small as it may be in some instances, we have a strict no overselling policy. When you get a RoseHosting VPS or dedicated server, the resources are yours and only yours to use to their capacity.