[mark-r]

px units are not actual pixels, they're a synthetic approximation. I don't remember the exact formula but I'm sure it's affected by font scaling the same as everything else.

[wizzwizz4]

They're defined in terms of inches. I don't remember the fraction of an inch that a CSS pixel is.

[mark-r]

I believe it's 1/96, while a point is 1/72.