You have an accent, that is sure, plus your intonation by slightly it is is is based upon Chinese phonetics, for you is not perceptible (I do not feel I have accent too but i do).
I have a friend from Switzerland, he has almost not accent, in fact i thought he spoke like native, however he does have an accent, native people will distinguish it, you do not feel you have an accent, plus is rude specially in the US to say you have accent if you are Asian or a minority.
He told me his wife and daughters can pick it up (when they say you have an accent from being away from Texas is a nice way to tell you, you do have an accent
What you can not understand is the tongue and its related muscles do adapt to the phonetics of their native language, since Spanish is my native language, my tongue muscles are adapted to the vowels and consonants of Spanish, my ear also is trained to Spanish sounds, people develop an accent because we can not distinguish a sound from another and the tongue finds it hard to think how to produce it.
I can believe you have a very good English, after many years of been in the USA, my Swiss friend has a really good English too, but you have a foreign accent that is for sure, in fact i can assure you, you speak English using words that get you closer to Chinese Grammar, for me, for example i tend to use cognates that are both in English and Spanish in few words words that are easy for me to understand.
Further more there are ethnic accent, you can tell a person is Hispanic, even if it is native from one who is African american, in fact the different accent a language has is related to the people that adopted the language.
Cuban Spanish for example has phonetics of Southern Spain mixed with African phonetics, Mexican Spanish was highly influenced by the Native american languages and Argentine and Uruguayan Spanish is highly influenced by Italian.
American Spanish is farther apart from its British origin compared to Australian English (remember Australia has more links to England and its independence was several centuries after the US got its independence), people from the north east cost in the USA has been influenced more by British English, than Texan or Californian English that have higher influence of Spanish, and you can see it in the way R is pronounce.
