https://www.postgresql.org/docs/current/history.html#AEN187
Berkley POSTGRES (с 1986) → Postgres95 (с 1994) → PostgreSQL (с 1996).
«Many people continue to refer to PostgreSQL as “Postgres” (now rarely in all capital letters) because of tradition or because it is easier to pronounce. This usage is widely accepted as a nickname or alias.» ←→ «Многие продолжают говорить Postgres, ибо традиция и произносимее. Приемлимость «Postgres» в качестве синонима высока.»

Когда-то думали переименовать в Postgres: https://wiki.postgresql.org/wiki/Postgres
Но консенсуса не было: https://www.postgresql.org/about/policies/project-name/
> The name Postgres is an accepted alias for the PostgreSQL project. However it is an alias or nickname and is not the official name of the project. Per Dave Page, PostgreSQL Core Team:
>    Following recent discussions on a name change for the project, it has become clear that consensus within the community will never be reached. In light of this, the core team have discussed the matter in depth and decided that the project shall continue to use the name PostgreSQL, but accept the use of Postgres as an alias.

С употреблением Britain и Great Britain то же самое (но там Great добавили, чтобы остров Britain с Britanny не перепутать, оба от лат. Brittania).