I believe what you're seeing is the tape that's UNDERNEATH the seam. Yes, about 2", give or take, on either side of the seam. When heat is applied to the tape, the glue on it softens and holds the carpet together. I have the funny feeling that your carpet isn't on any padding. The tape can 'raise' the carpet on both sides of the seams. And as others have said, the lighter the color and the shorter the knapp, the more likely you'll see the seam.
Do talk to the installer again, or as I think I've suggested before, have another pro take a look at it. OR, better yet, have the Salesman come take a look. After all, he/she IS the one that sold you the stuff, eh?