This happens to soooooo.... many people who buy a GLOCK as their first pistol. I wish you were at the same range as me. I could have helped you save a lot of time, money and frustration just by getting you to try a GLOCK with decent sights installed and/or showing you how your own pistol is sighted in now.
Get rid of those ridiculous 5 dollar plastic sights that came with your pistol. Those are not even real sights in my view. They are just meant to keep the sale price of the pistol down and then be discarded by each noob who buys a GLOCK and realizes that they are useless. Outside of film and televison, you will never see any police officer, military personel, competition shooter or anyone that knows what they are doing use those. I'm sorry to hear you wasted 2,500 rounds of ammunition on those sights ($750 at 30 cents a round in 9mm!!).
If you think this kind of sight picture is dumb, then I agree with you, but that is about how your GLOCK, and all others that still have those plastic "U-notch" sights are sighted in at 10m. If you think of the top of the rear sight being right at the middle of the front dot, that is how they shoot to point of aim. If you level the top of the rear sight with the top of the front sight you will shoot low every shot. The further the distance the lower the shot. At 10m you will be about 3 inches low, and about double that at 25m.
If you want to use a sight picture like this, the way everyone in the world expects it to be, you will have to buy new sights and install them. (Picture is of a different kind of pistol, but just for illustrative purposes... couldn't find the right photo in my photobucket account).
These are some Trijicon sights on a GLOCK 19. The 9mm/.40S&W sights are the same. The .45ACP sights are different.
You will be surprised how accurately you can learn to start shooting your GLOCK when it actually lines up the way you are expecting it to.
You should be beaning those metal plates at least 9 shots out of 10, even as a novice. 2 out of 10... even bad technique is unlikely to explain that. You've got a fundamental disconnect/misunderstanding about how your pistol works.
You could learn to shoot it the way it is, but I wouldn't. It is much slower to try and line up the top of the rear sight with the middle of the front dot... and your brain is clearly thinking in terms of levelling the sights anyway. Good luck.