Well, first off, I made a mistake. Flight 175 was a United Airlines 767-222, not an American Airlines 757. (not much difference in planes tho)
Photograph of the plane just before it hit the WTC.
The bottom of this fuselage is not smooth like the ones in the photographs of the 767s at the bottom of this posting.
I've made an arrow pointing at the bulge on the bottom of the plane that is inconsistent with the bottom of the United Airlines 767 in the below photograph.
Here is a photograph of a United Airlines 767:
The bottom of the fuselage is smooth.
Bottom of another 767. Again, notice that it's smooth and without bulges.
Basically what all this boils down to... what I'm essentially trying to say is... blame Canada.