Welcome to a highly regulated industry with very specific terms used inconsistently.
I'm trying to avoid confusion if people actually visit the website or contact the airline. The airlines refer to both pieces as carry-on luggage in their CoC and official documents, the term personal item is only a vernacular that appears in the FAQ type sections. This is important to understand if you actually contact the airline past their teir 1 customer service number or read the CoC to determine what's permitted. Yes, I know that the CoC is a bit like reading the iTunes T&C, but higher tiers in customer service do use the terms correctly, as explained below:
It's not that they may both be, "Carried on," Carry on luggage is actually a term for luggage that is permitted on board the aircraft with the passenger that is not included in the FAA exempt list of personal items that you must be permitted on board. The FAA exempt list are what are officially considered personal items, and include food for consumption on the aircraft, reading material for your flight including tablets or a small laptop sleeve, medical devices including eyeglasses and contact lenses and even some toothpastes and bottles of water, a pair of binoculars or a camera that you hand carry on (usually around your neck), sunglasses, jackets, feminine hygiene products, and other sundry personal items. Legacy and EAS airlines also add a few more items from old hangovers in CAB agreements, such as extra winter clothing (socks, hats, scarves, and gloves not being worn) for airlines operating in certain regions, as well as purses or man bags below a certain size. Specifically personal items as defined by the FAA are things that do not go in a bag and are hand carried or on your person.
The British English I mentioned only because some airlines that fly domestically list it as such, especially ones with large UK/Commonwealth overseas presences, and also because some people from these boards are overseas travelers. Additionally, a smaller piece is sometimes referred to as a, "Cabin bag," indicated its intended use for things you wish to access during the flight.