My guess is its because that is what is in your document.
See, I'm trying to make a certificate with signatures pre-printed on it. I was trying to make a transparent background to the signature so that I can place it on different house color. When I print it out the certificate, the "transparent" background of the signature isn't transparent, it looks like it have 10% opacity on top of the background. I have even tried to fill the "transparent" area with the same background color as the certificate, but the patch of tint still shows on the print out. Does anyone knows why?