The official policy of SE is to give all sites the descriptive name and *.stackexchange.com domain because it is good for SEO and SE business overall; there are basically 3 reasons for an exception:
- being a pre-Area 51 and pre-SE site (StackOverflow, ServerFault and SuperUser),
- political reasons (MathOverflow, AskUbuntu, +-AskDifferent),
- issues with a descriptive name or a strong community demand (Seasoned Advice, Mi Yodeya, Arquade and CrossValidated).
In our case, the reason to be in the group 3 is that Statistical Analysis does not cover the whole scope of the site (mainly machine learning, but also visualization and data mining; and it is a real problem what numerous failures of machineLearning.SE have shown), and is on its own a historical artifact because this site was proposed on Area51 as only for statistics and expanded during the private beta period. There was even an idea to change the descriptive name to "Data Stack Exchange" but the data.stackexchange.com have been already taken by the Data Explorer.
Nevertheless, because of the already mentioned SEO stuff and maybe some internal mechanics sites from the group 3 still have descriptive name.stackexchange.com domains and use their descriptive name in data dumps and darker places as the data explorer.