Now, before you click away, I know what you're thinking, how can the default 3D View be tilted? This guy must have messed something with his camera, or activate the trackball views settings by mistake right?
Well believe it or not, the default 3D view (as in, the view of the default 3D viewport in the blender startup file) is tilted by around 0.8 degrees.
Here is a quick proof I just threw together, I believe it's important for you to watch it until the end, as the main proof really comes around 0:40.
As you can see, when trying to align your view with the cube, it doesn't lined up, this can be seen by the jagged selection edges, and of course with the Camera aligned to view.
You can also see at 0:35 that when switching to the Front Orthographic viewpoint the view gizmo itself is rotating on the X axis a little bit.
And of course, when switching back to the normal Perspective view, the viewport view is realigned, enhance why the Camera that's been re-aligned with the view is now sitting properly on the X axis. (see 1:10)
Now, why is this happening?
After a bit of investigating, I discovered that the issue as been around since Blender 2.5, which is now more than 10 years ago
Blender 2.5 was the first version to have the default startup file view not lined up with one of the orthographic axes. But it also had the Trackball orbiting style as default instead of Turntable. This might explain why, 10 years ago, a blender dev might have tried to get the view somewhat lined up in trackball mode, and got it almost right.
Here's a quick demo of the same problem on 2.5 Alpha 0:
Now, while this isn't a top priority issue, this as been around for quite some time now, so this is not really an inquiry for an immediate fix, but more of a way too long report I did since I couldn't find anything on it online, and it drove me crazy for a few hours when I first discovered it.
Broken: 2.5 Alpha0 and beyond
Worked: 2.49b, where the view was orthographic by default
Exact steps for others to reproduce the error
Start Blender, use the viewport (Will disappear if snapping to an Orthographic axis)