Why would a guy say he was seeing someone if he was not?

This guy and I have been flirting for months. He never took it further, so I got brave and asked him out. He said no because of an age difference. We kept talking and a little later in the conversation he said he was seeing someone. I am almost 100% sure that he isn’t. This was a few weeks ago, and he still has been flirting with me. I am so confused. If he is not interested, why would he not just leave me alone and act indifferent around me. Did he say he was seeing someone as an attempt to hide feelings?

Why would a guy say he was seeing someone if he was not?
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