Font size discrepancy in .NET GDI+?

Question

I am wracking my brains in trying to understand the discrepancy between the font sizes users select or specify (for example, using a FontDialog) and the em-size

Answer 1

there is no pattern in the discrepancy

As you can see, font sizes happen in increments of 0.75.

Edit: You can fine tune sizes if you do not use the font dialog, but the results I suspect will be less pleasing than the 'preferred' sizes.