Most unicorn companies don't start out by owning their best, exact-match .COM domain name. When a company starts its life, domain names aren't necessarily a priority, so many opt for lesser names. Lesser names could include a prefix word (such as "Get" or "Try"), or they could be an alternative extension to .COM (such as .CO or .IO).
On their way to becoming a unicorn, many of these companies paid the sometimes vast amounts of money needed to acquire their exact-match .COM domain.
Domain name upgrades are fairly common, but there are very few resources available with a specific list of companies that have upgraded their domain.