I didn't know better

[frindle]

Hello. Would you tell me what the underlined phrase means in this context?

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"Oh, my God, they know English. They speak perferct English," said Lexi.
"Well, duh. They live here, work here, most likely go to school here. They
had to know some English," I said.
But the truth is I was just as surprised as she was.
Unlike Lexi, I was ashamed that I didn't know better.

(Here they means a Chinese family who runs a Chinese restaurant in their neighborhood.)

[JohnParis]

Unlike Lexi, I was ashamed that I didn't realize that the family would speak perfect English for the reasons I just gave above.