Conversations with an American couple of Chinese descent with the last name of “Wang” confirmed my understanding of the pronunciation, but I am certainly no authority, so take this with a grain of salt.
China has always relied on written characters that were not based on pronunciation to unite a surprisingly diverse country. People who speak mutually unintelligible dialects can still read, write, and communicate by characters even though they pronounce them very differently. Mandarin is now taught in schools as the official language of China, but many Chinese continue to speak their native dialect.
Pinyin is the official phonetic system for transcribing Mandarin pronunciations of Chinese characters into the Roman alphabet and is based on romanization developed by early European missionaries. These missionaries naturally used the Latin pronunciation of the alphabet, which is different than typical American pronunciation.
In Pinyin, “a” is pronounced as in “father,” so “Wang” rhymes with bong, not bang (approximately). Some early Chinese immigrants must have grown tired of hearing their names mangled and found Americans pronounced it correctly if they spelled it “Wong,” and so they did. Hence, “Wang” and “Wong” refer to the same family name and both should rhyme with bong.
Pop quiz: How should you pronounce the Tang dynasty? Hint: Not like the orange drink.