Yes. In order for all of the existing mechanisms for managing things that look like apps to work in reasonably logical ways (launching the app, switching between apps, resource managing the app while it is in the background, isolating faults from the app, etc.), home screen web applications are each represented by Web.app.

On a similar note, in order for similarly interesting behaviors to work with iAd (home button acting as "close", address space isolation to keep your process from messing with the ad, etc.), there is an application that backs them called WebSheet.app that also causes these to be in separate processes.

Of course, this means that there is no real good reason why Apple has disabled Nitro for full-screen web applications (that I at least yet understand), and so I'd expect that this was frankly just a silly oversight (and I may know more about what reasoning went into this when I get around to pulling apart how Apple grants MobileSafari this privilege).