People responding to my Arabic in English

I'm Arab born/raised in the US and speak Palestinian dialect.

I noticed that sometimes when I'm at an Arabic restaurant/store the staff will exclusively speak to me on English, but will speak to other people in Arabic. I really can't understand why. This is after I greet them and ask questions in Arabic. It doesn't happen often, maybe every 1/30 times I speak Arabic to a stranger.

For example at the Arabic grocery store, I asked an older man

مرحبا كيفك بتعرف وين الحمص

He responds: "Yes it's in isle 4" then turns to his coworker and says

يا علي جيبلي السكينة

or sometimes I'll go to an Arabic restaurant, only speak Arabic to the waiter, they'll respond in English to me then only in Arabic to the table next to me. They'll keep doing this even though I only respond in Arabic.

I don't understand this at all. Any insight would be appreciated.