Xorg.conf DPI settings VS. fonts DPI settings - Confusing!

Question

I honestly have to admit, that I don't get it. Having a ThinkPad T42p with a 1600x1200 LCD with the visible dimensions of 304 x 228 mm, I have set that in xorg.conf ("DisplaySize  304 228"), which results in Xorg setting up 133x133 DPI on start (without specifying this DisplaySize setting, it defaults to 75x75 DPI).
133 DPI results in fonts displayed way too large, although I remember WinXP (which came preinstalled with this laptop) setting 133 DPI as well, which also gave large fonts, but not that large as seen with Xorg.
I am now using KDE (specifically funkyou's KDEMOD), and I have set "Fixed DPI: 96 DPI" in font-settings, which means, text is displayed in a reasonable size with 96 DPI, but Xorg is still set to 133 DPI:
$ xdpyinfo | grep res
resolution: 133x133 dots per inch
I really don't understand this font vs. Xorg DPI setting. How does this fit together?
Why are fonts shown way too large at 133 DPI, if that's the correct DPI setting for my screen size & resolution?

Answer 1

Why does it need to scale properly, if Xorg should already have set DPI to the correct value calculated from the display size?
How did you determine, that 100 DPI is the correct fonts-DPI-setting for your display?
This is just weird, because setting the DPI setting for fonts does not influence the DPI setting of the X server (that's what I mentioned above, selected "Fixed DPI: 96" in KDE's font-settings, and xdpyinfo still reports 133 DPI).
Concerning my LCD, 133 DPI seems reasonable for my 15" laptop-display with relatively high native resolution of 1600x1200, so the Xorg setting surely is correct. But why do I then need to set 96 DPI for fonts to be displayed in a proper size?
Even more confusing is, that the settings for the ATI Radeon open-source-driver offer even more options concering DPI:
#Option "MergedDPI" # [<str>]
#Option "ConstantDPI" # [<bool>]
As seen, I have not enabled these, but that makes altogether 4 different settings for influencing DPI.
Fact is, 96 DPI in KDE's fonts-settings seems to lead to the desired result, although that value should not be the proper one for my display-size & resolution, as well as I have no idea, what the X server resolution (133x134 DPI) is all about, especially because it does not seem to be of any importance if font-settings are forced to a different DPI setting (which means, it does not seem to matter, if the X server resolution is set to 75x75, 96x96 or 133x133 DPI, as long as the option for force fonts at 96 DPI is selected in KDE's fonts-settings).