My boss confronted me about only working 7 hours and 45 minutes a day, what do I do?

Hey all,
Today, my boss called me into his office and confronted me about the fact that I take a 45 minute lunch when I should only be taking 30 minutes. I work in an engineering office and we can take as long a lunch as we want whenever we want, as long as we are working 8 hours a day. I get in at about 7:30 and leave at 4 everyday, which totals 8 hours and 30 minutes, and each day I take around 45 minutes to eat lunch. So technically I come 15 minutes short everyday. Some ask why I don’t just take an hour lunch like most of my coworkers, but I don’t need a full hour, so why would I want to leave later? I get all my work done on time or early, and I often find myself with no work to do. The whistleblower that told my boss about this is an older lady that can’t get her work done in 8 hours because she’s bad with computers. She was upset that I get to work after her and leave before her. My boss said we would discuss this tomorrow, and I don’t know what to say. Fact is, this whole situation is about office politics. I’m a salaried employee and if I don’t get paid to work overtime, I’m not gonna waste my time when I finish early. How can I tell my boss this without coming off as arrogant or entitled?

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