Screen Scratchability - Getting Poorer Since 3GS

I use my iPhones without screen protectors or cases, but I make sure to avoid it getting knocked or scuffed up by putting it in a pocket without keys, coins etc.

My 3GS had virtually no scratches on the screen after being used for a year. I lent it to someone (who's generally rather rough with gadgets) for another year, and it came back with a few very minimal scratches on the screen, virtually unnoticeable until a bright line is shone on it.

Having the good experience of a near scratch-proof screen, I went without a screen protector on the iPhone 4. This time, there were noticeable scratches on the screen. The 4S was worse, with a couple of scratches deep enough to be “rainbow lines” with the display was turned on.

I have been using the iPhone 5 very carefully since launch, and the aluminium back and chamfered edges are pristine and I expect it to remain so. Not really sure how people can get scratches on the metal because I've had very good experiences with Apple aluminium products (iPod mini, PowerBook, MacBook Pro) and the iPhone 5 does not seem different. The screen is a different story though, as in less than 2 weeks there are many long hairline scratches on the iPhone 5, seemingly half as bad as the 4S now, and I have been using the 4S for almost a year, and with much less care (I've dropped the iPhone 4S several times and forgone my no keys, coins rule after seeing the scratches on the screen).

Interestingly enough, majority of the scratches are focused on the right side of the screen where I use my right thumb to scroll.

My friend's iPhone 5's display has circular scratch marks virtually all over the screen.

I believe reading that Apple moved away from Corning's Gorilla Glass at one point; there were mixed reports of the kind of glass display iPhone 4 was using. Some said it was Gorilla Glass while others mentioned another kind. Whatever it is, I think the scratch-resistance has really gone down tremendously after the 3GS. Anyone with similar experiences?

(To be precise, most of the scratches are visible when the display is at an angle, catching a light reflection.)