It's because you're in a high signal area. Go somewhere else where you have less bars to begin with and do the same thing... you'll see the bars drop. For example, here in my home I get 5 full bars of reception and no matter how I hold the phone, the bars will not drop. However, I went out and about today and there were several places where the bars would consistently drop every time I tested holding the phone this way. One area where I only got 2 bars to begin with dropped to "Searching..." every time I held it this way.
To repeat, a strong signal area may not be affected by this problem, but lower signal areas will. That said, holding the phone normally does not cause the problem on my phone - only when holding with my left hand when I cover both the top and bottom frame and when I'm in a lower signal area. Which means, I would probably never experience it in actual use. Plus, when I was done testing, I put a case on it anyway for protection.
