It's "straight" insofar as it's parallel to the deck but your noggin is above is so you're looking down onto it - thus it's "sloped" from your perspective.
FWIW, a slightly more irritating example of this "issue" is the way stations and ships look in the HUD.
If you align yourself so that, say, a station looks "square-on" in the holo, it'll actually be skewed away from you.
Conversely, something that IS squarely in front of you will look skewed in the holo'.
It's all 'cos of the vanishing-point perspective that the holo's use to draw images, coupled with the fact that they're either side of your noggin.
Now that IS one that bugs me - lining up the mailslot.
If the hologram shows it aligned dead center, then damn it, that's where we should be dropping in. Oddly, it IS possible to actually do this, but there's some secret rubic's cube approach you must make for it to happen.