1. The definition for the two genres are almost identical: An evolution from the more rudimentary three-chord garage punk, with an interest on experimentation.
2. Just about every Art-Punk album is also tagged as Post-Punk

So what on Earth is the difference? Just seems like two different people came up with each term and then people didn't decide to settle on one, continuing to use both.

I can vaguely surmise some sort of a difference in that there are some Post-Punk albums not also regarded as Art-Punk, like Joy Division's Unknown Pleasures, which leans more Gothic Rock, but almost every Art-Punk album is also tagged Post-Punk, so maybe it's this:

Art-Punk is more concerned with being groovy and rhythmic, whereas Post-Punk is more interested in being moody and atmospheric? That could be a difference, but I'm not sure. What do you think?