One reason is that if you load the web page, you have to download the UI every single page load. With an app, the downloaded data should be quite a bit more stripped down... but to be honest I couldn't say how the app gets its data.
It might be scraping downloaded pages. In that case, I'd tend to agree with you about an HN app as it doesn't contain anything that requires a fluid interface.
"download the UI every single page load" - Client side XSLT resolves this. I'll admit that client side XSLT has other problems though. Compare these two urls:
No. Any well written web page can cache itself for offline use, and use AJAX to load all data after the first page. This is how iUI works, and therefore most iPhone-specific web apps.
