There's fallback logic which is fairly involved. en.xml contains the data for American English, hence it has short names for American zones. Most English variants fall back to en_001.xml first, which is "world English" with British spelling (en_GB has country specific data for the United Kingdom, so it shouldn't be in the fallback chain for other countries). en_001 and other locales that fall back to en block the fallback to American short zone names (https://github.com/unicode-org/cldr/blob/main/common/main/en_001.xml#L983).