Default length unit with PGFPlotsX and Plots

I am using PGFPlotsX as a backend for Plots. I have specified the plot width as 800. After I saved the plot to pdf and inspected the properties, the width seems to be 194mm, which is around 550 pt.

What are the default units of length in PGFPlotsX? Is there a way to change them?

The default unit for plot size is px and according to the source code 100px should equal to 1inch.
Which seems to hold only roughly in this case.

Are all the other sizes also measured in px? If I specify the fontsize with PGFPlotsX, is it in pt, or in px?

fontsize is in pt