I've had this before too. It depends on the type of question you're asking. If it's just a general question about something that doesn't require access to your account/booking for specific information relating to you, then they can answer it without needing to clear DPA. If your last question was more specific and they needed to access some of your info to answer it, then they have to come back and ask the security questions.

Seems logical to me as I wouldn't want them to access my personal details for a general question that could be answered without having to provide all of my details.