Opacity on .png fluctuates with light/dark colors

I've been having a persistent issue where I just want something (say a .png) to be a fixed transparency, say 75%, and it looks acceptable in preview, but after I've rendered and exported the file the video is visibly different.  In preview, transparency affects the whole image; it's 25% less visible, but after rendering, the lightest parts of the image haven't become transparent at all and the darkest parts of the image almost entirely fade into the background.

I wanted the WHOLE image to be 75% opacity, not select parts of the image.  I know Blend Mode can artificially create this effect (such as with Multiply or Screen), but it's set to Normal and this is still happening.  What is wrong and how can I avoid it?