I'm installing baseboard for the first time and I keep noticing when doing 45° cuts the top is sticking out a fraction more than the bottom, see picture. I'm using a dewalt 716 miter saw and have used multiple tools to test and the saw in tuned in properly but still am having this issue.
Not every corner is perfect, and some adjustment is needed for every cut in some cases.
Either the cut is not square, and the saw is still not adjusted, or the wall is bumped out at the bottom. If that is the case the wall can be carved back behind the base or the base can be thinned down on the back to get the corner to come together.
The drywall corner bead (the angled metal corner reinforcing strip that covers the whole outside corner) changes your angle at the corner, even if the walls are square (which they likely are not). You need to miter both molding pieces to a bit more than 45°, like a 1/2° on both to start, judging from your photo.
You've checked both axes of the saw, and when cutting have made sure the stock is flat against the fence, right?
If the problem is just this corner, I'd be inclined to assume thete's a bump or curve in one or both of the walls pushing the bottom of the joint outward. Or perhaps the floor slopes down a bit toward that corner so the angle really is tilted this way.
Suggestion: If you can't find an obvious cause, and it isn't happening at every corner, either pull that inward to close it, whittle it to fit, or fill the gap with wood putty and trust that nobody will ever notice aftef it's been painted.
In any house, you can pretty much assume that square and plumb aren't exact. Do the best you can.