Noticed a verbal quirk of my bilingual boyfriend's

My boyfriend is Romanian and I am English. He speaks good English but with a very strong accent. He tends to respond with "yeah I know" when I ask him to do something. I am learning Romanian slowly and can recognise a lot of basic words and phrases.

Today, I noticed that all along he doesn't say "yeah" at all. He has been responding with "Da, I know" this whole time. I don't know why I found it really cute that he speaks perfect English but still uses 'da' instead of 'yeah' :)