Ask HN: Interviewing, what did I do wrong?

I submitted an application to a company for a software engineering internship, and was contacted by one of their recruiters.

I was interviewed by him on the phone, then scheduled for a technical phone interview, which I passed.

I was then invited on site for 3 1-hour technical interviews, which I passed.

Then I was invited back for a 30-min interview with the head of engineering. He asked me a few questions about scalability of platforms, my favorite platforms, my preference for front-end vs backend, and then asked how much I make per hour at my current job (which is $2/hr more than glass door says their interns make). We finished the meeting, he shook my hand, and 3 days later, I got a no-reply@company.com email stating I had been denied.

What did I do wrong? I'm honestly baffled.