Yeah I would expect a different result when using a material and pulling from the alpha channel. You are only using the information from the single channel of the texture instead of the entire RGB values output as an image.
My results will also vary because of my native computer resolution and the bits per pixel of the texture sample I was using. Using a material is not a bad idea as it can give you (as you recently discovered), greater control over images. If you are getting great results with this, I would use this approach! Either way I am glad you were able to learn a little bit and get your issue to resolve after trying various approaches.
If you have any further questions please let me know.
Cheers,