I think this means HN readers are way more 'engaged' than TC's. HN readers are more interested in the topics being treated here, and click-through rate is probably higher. It's also important the fact that in HN you only have a link to the site, not an article explaining what the site is.

Oh, and I also agree with gojomo: would be very interesting to have followup data, most signups, most active signups...