export – Why does Mathematica set a resolution when exporting a PNG file with given pixel size?

I use the export command


to create PNG images with transparency and some fixed dimension in pixels (1280×960). Strange enough, Finder (I am on macOS) shows me a “resolution” of 144 for the exported images. Indeed, I can add the Option ImageResolution->integer in the export command and if I do so, Finder afterwards shows me the chosen integer as the “resolution”. This makes no sense to me as the images have the same dimension in pixels and have the same file size. In principle, I could just ignore this issue, but unfortunately it matters when I import the pictures into Keynote. If I import a 1280×960 pixel image with a “resolution” of 144 into Keynote, its “Original size” according to Keynote is 640×480.

Does anyone know what’s the logic behind all this?