Confluence PDF exports can't achieve true font-size of 11px

Font size not as expected in CSS for PDF Exports, for example 11pt is actually 10.66pt. The following sizes were tested:

10 -> 10
11 -> 10.66
12 -> 12
13 -> 13.33
14 -> 14

The assumption is that there are some rule which is preventing the expected output.

This is only found in the finished pdf.

There is no sign of this behavior in the CSS nor HTML. Using custom CSS cannot override the behavior.