Tzon, this is an excellent observation!
Here is another set of tests that show this phenomenon. FYI, Word 2013 uses the grayscale antialiasing method (the one that cannot be turned off neither be tuned). Wordpad uses the ClearType antialiasing method (so here, depending on your settings, my
tests might appear different on your wordpad, but not the png's I posted).
The text is a series of capital i's, in Calibri, in increments of 0.5 points until the right edges appear aligned in Word. The text is copied/pasted in wordpad. It is interesting how much the right edge is jagged in Wordpad and it is obvious that Word
does a much better positioning..
Notice the change in the right edge alignment and the repeating (moiré?) patterns that are more visible in small point sizes.
Also, view the images at 100% scale and try to change the vertical viewing angle. On my monitor, the grayscale text, appears, less or more contrasty when the vertical angle changes, without color bleeding. Also, with grayscale, edge effects (whiter or
darker edges) are more apparent when the vertical angle changes. The ClearType text periodically shows color effects (green/magenta bleeding) at increasing point sizes, which are inverted when viewed with the opposite vertical angle. At the optimal viewing
angle (perpendicular to the center of the screen), the color bleeding is apparent when viewing the text at the top or bottom of my 18" Sony LCD screen. The non-ClearType text in Wordpad, shows no difference when changing the vertical angle. Although in this
case, ClearType is turned off, some grayscale anti-aliasing appears in Wordpad, which I don't know where it comes from.
IMHO, the best rendering method is the grayscale method used in Word, but only if it can be tuned to the specific monitor/viewing angle used. Otherwise, ClearType has very good results, if you do not deviate much from the optimal vertical angle.
Word 2013 Grayscale with subpixel positioning on and optimized for readability (Options/Advanced/Display):

Word 2013 with subpixel positioning on and optimized for readability:

Word with subpixel positioning off and optimized for layout:

Word with subpixel positioning off and optimized for readability:

Wordpad + ClearType:

Wordpad, with ClearType off:

and, btw, this is the subpixel layout of my monitor:
