I've seen this in my own work. It was usually because of non-pixel-perfect coordinates or scaling.
Looking at the image you posted, the text seems kind of blurry.
Have you tried hitting the "Make Pixel-perfect" button?
If that text is multi-line, is the line spacing correct?
If it's vertically center-aligned, it might not be pixel-perfect, depending on the number of lines.
You might have to get a script to call MakePixelPerfect() or whatever the function is, after you've set your button text.