Anti-aliasing and font-hinting is most likely the issue.
Live type anti-aliases differently than other objects. And positive objects anti-alias different than counters or negative objects.
Here's what I merely suspect is taking place....
- White type on black background: the type is anti-aliased outward. So you get white anti-aliasing to a black background. This visually makes the white appear a tad heavier at its edges. If the type is small, 12pt or less, the live type also may benefit from font hinting within the font file itself. Hinting tells the app how to draw the character at smaller sizes. The hinting issue is a guess, you haven't indicated what size you are working at.
- Outlined type on a black background. Now you have a white object on a black object. So, the anti-aliasing works both ways. The app attempts to "blend" the edges of the top object, so you get more "grey" in the anti-aliasing. This will result in visually the white looking less "heavy".
- Counters, or cutouts on the black. At this point the anti-aliasing is now black and outward. So, for counters, or holes, you get black anti-aliasing outward filling in the "hole" a bit. There's no more white in play, it's all black and shades of grey. This means the counters appear even less heavy.
What to do.....
Work larger. An easy solution is to work larger then reduce. If you start with larger type sizes you may not notice the anti-aliasing as much. This will let you disregard how it may be altering the visual a bit. Then reducing to necessary size will still do the same thing for anti-aliasing, but it should be consistent across objects.
You could turn off anti-aliasing in Illustrator. Of course, then you'll get stair-step curves and the art won't be as smooth visually, but you won't see any changes due to anti-aliasing.
- Ignore it. If each image will ultimately be the same size and configuration, then just pass over the variation and set up all your images. In the end if they are all configured the same and constructed similarly, they should all appear the same. Because they will all be affected the same by anti-aliasing.
- Upgrade your monitor. Yeah, this is a bit ridiculous, but the reality is the more resolution/depth your monitor has the less you'll see anti-aliasing. Things like a 4K or retina displays won't show the same variation with anti-aliasing as a standard monitor will. It will still be there, you just wouldn't see it as much.
This is all just my "best guess" without seeing the file itself.
My immediate inclination is that the type may be very small in your file and that is why you are seeing such a wide range in this. Setting 24pt type, outlining it, subtracting it from a background and then reducing it almost always results in a very slight difference opposed to starting with 6pt type and doing the same procedure. And some fonts themselves may have more of this issue than others, especially if font hinting is in play.
The easiest solution is to work larger and reduce to final size in most cases.