The barrel assembly, which includes the front sight block, is slightly out of alignment with the upper receiver.  This is a common problem due to folks not supporting the barrel properly as they tighten the barrel nut, or if they use barrel blocks instead of receiver blocks to install the barrel.

Anyone who has the tools to rebarrel an upper should be able to fix this for you.  If you look carefully, you should be able to see that the front sight is leaning a bit in one direction (the same side that your rear sight is bottomed out against).

-Troy