Some of them are different. And constant traps are different, the lemming is technically still there while the animation is playing. On non-constant traps the lemming is removed as soon as the animation starts.
Maybe the number would help? I'll go count again to make sure I have it right.
EDIT: Some traps have trigger areas only 4 pixels wide. I missed a few, cce may be right.
EDIT2: cce is right, just ignore that question. The problem was that objects are aligned to multiples of 8, so some of them missed.
An interesting thing I discovered though is that the trigger area for all non-constant traps is 4 pixles wide. I think it's also 4 pixels high.